He is probably guilty of negligent homicide or involuntary manslaughter, at most (a minor felony), and is possibly not guilty of a crime at all.
The primary distinction between classes of homicide is mens rea (i.e. intent).
The only affirmative act he took was to move the pillow. He did so both without intending to or knowing that he would kill Jane (the intent necessary for murder), and also, without clear actual knowledge that he would be creating a risk that Jane would die (a reckless state of mind that would support a conviction for manslaughter).
Also, note that Walter himself, at this time, is not engaged in a felony, so he is not guilty of felony murder. We can presume he is present with the consent previously given of the owner of the property and did not mean anyone any harm. Likewise, this is not what is meant by "extreme indifference" for purposes of a murder statute, the paradigm of which is shooting randomly into a crowd knowing that someone will almost certainly be killed without knowing or intending that any particular person will die.
The mental state necessary for negligent homicide is the equivalent of "gross negligence" in a civil case and is called "criminal negligence" in a criminal case. To be criminally negligent a person must fail to perceive a substantial and unjustifiable risk that a certain result will occur, and the risk must be of such a nature that the defendant's failure to perceive it constitutes a gross deviation from a reasonable person's standard of care. If a jury found that a reasonable person ought should clearly know that moving a pillow put Jane at risk of dying, then he might be guilty of criminal negligence. But, if a jury found that a reasonable person would not know that moving the pillow put Jane at grave risk of death, his action would not be criminally negligent.
There are also at least three questions of causation which is not entirely independent of the question of negligence.
First, generally an act is only considered a cause of a consequence if it is a foreseeable result of the action. If Jane's vomit caused death is not a foreseeable result of moving the pillow, then her death might not be legally caused by moving the pillow.
Second, how likely is it that she would have died even if Walter had never entered the room. There are lots of ways that the pillow could have been jostled during the night leading to the same result. If it likely would have happened anyway, Walter's involvement might not be the legal cause of the death.
Third, how much fault should be attributed to Jane? This is closely related to the second question. If her death was primarily caused by her getting dangerously high and placing herself in a vulnerable position, perhaps Walter's involvement is not a meaningful cause of the death.
A New Mexico court has held that the defense that the victim was negligent has value only if it establishes that the victim's negligence was the sole cause of the accident. State v. Maddox, 99 N.M. 490, 660 P.2d 132 (Ct. App. 1983). But, what about Jesse's negligence? Under a relevant standard criminal jury instruction in New Mexico:
The State must prove beyond a reasonable doubt that the defendant's
act was a significant cause of the death of __________________ (name
of victim). Evidence has been presented that the negligence of a
person other than the defendant may have contributed to the cause of
death. Such contributing negligence does not relieve the defendant of
responsibility for an act that significantly contributed to the cause
of the death so long as the death was a foreseeable result of the
defendant's actions. However, if you find the negligence of a person
other than the defendant was the only significant cause of death, then
the defendant is not guilty of the offense of __________________ (name
Caveat: A number of states impose strict criminal liability on drug dealers, often for murder, if someone died from using a drug sold by them, but often it has to be a child, and often the drug has to be the proximate cause of death, e.g. due to an overdose or impurity in the drug. I would presume that Jesse and not Walter supplied the drugs to Jane, that Jane is an adult, and it is not obvious that the drug itself (as opposed to the vomiting due to the manner in which the drugs were used) was the proximate cause of death, so a statute like this might not apply in any case.
This brings us to the hard part of the question:
Without the pillow Jane rotates on her back and starts to vomit and
cough, still sleeping. At first Walter tries to react, running to the
other side of the bed to help her, but then he stops and decides to do
nothing as she dies.
Note that if Walter had moved the pillow without knowing that he was creating a risk, left the room ignorant, and then this happened, surely Walter would have no legal liability for Jane's death.
If Walter develops the necessary intent for criminal liability, this probably doesn't happen until he observes that she is starting to choke on her vomit and might die. Even then, he does not intend for Jane to die and probably doesn't even know for certain that she will die from his inaction, so he is probably, at most reckless, if he has a duty to rescue for criminal law purposes.
Generally, under both civil and criminal law, there is no duty to rescue, even if you can do so without any risk of harm to yourself.
But, there is an exception, at least in civil liability, for a duty to rescue that arises from the fact that you put the person at risk of peril through your affirmative actions.
Does this apply here, at all, or in a criminal case?
The first question is the exact language of the homicide statute. Some homicide offenses require affirmative acts, while others can arise from acts or omissions where there is a legal duty to act.
Every crime requires some voluntary act or omission, and the voluntary act itself was not a crime and perhaps was not even a tort, at the moment it was taken, because Walter did not realize that his act created a risk of harm. He create a peril, but he did so innocently.
A pretty standard formulation is that an omission is only a crime when the law creates a legal duty to act, but this is, of course, a question begging standard as it doesn't clarify whether there is a legal duty to act, which is at issue here. As the previous link notes, creation of a peril can give rise to a legal duty to act, but only sometimes.
(4) Duty arising from creation of peril. If a person acts culpably to
imperil another, he or she has a legal duty to rescue the victim.
The cases are split on whether a duty to rescue arises if someone
innocently or accidentally imperils another.
This case would fall in the category of someone who innocently or accidentally imperils another, in which the cases are split, which which the linked article cites the following authority:
Compare Commonwealth v. Cali, 247 Mass. 20, 24-25, 141 N.E. 510, 511
(1923) (defendant under duty to try to extinguish a fire that he
accidentally set to his house and thus was guilty of arson when he did
not) with King v. Commonwealth, 285 Ky. 654, 659, 148 S.W.2d 1044,
1047 (1941) (defendant who, in lawful defense of a third person, shot
and wounded an attacker was under no duty to seek medical attention
for the wounded assailant).
A commentary that is part of a California standard jury instruction (for involuntary manslaughter, not murder for which this kind of liability is presumably not available) makes the following observation:
A legal duty to act may also exist where the defendant's behavior
created or substantially increased the risk of harm to the victim,
either by creating the dangerous situation or by preventing others
from rendering aid. (People v. Oliver (1989) 210 Cal.App.3d 138,
147-148 [258 Cal.Rptr. 138] [defendant had duty to act where she drove
victim to her home knowing he was drunk, knowingly allowed him to use
her bathroom to ingest additional drugs, and watched him collapse on
the floor]; Sea Horse Ranch, Inc. v. Superior Court (1994) 24
Cal.App.4th 446, 456 [30 Cal.Rptr.2d 681] [defendant had duty to
prevent horses from running onto adjacent freeway creating risk].)
These examples would suggest that an innocently or accidentally created risk is sufficient to create a duty sufficient to support involuntary manslaughter liability for an omission under California law, and would probably lead to involuntary manslaughter liability in the case in the question as well, under California law.
New Mexico, unlike California, does not have a standard criminal jury instruction or really definitive section of its criminal code that clearly resolves this question, although the fact that California which uses a murder, voluntary manslaughter, and involuntary manslaughter distinction in the same way that New Mexico does, limits criminal liability for omissions to involuntary manslaughter suggests that New Mexico would as well.
The New Mexico case State v. Greenwood, 2012 -NMCA- 017, 271 P.3d 753 (N.M. App. 2011), touches on the issue, suggesting that there may be liability only for involuntary manslaughter (or certain specialized crimes based upon a relationship such as that of a nursing home to a resident of a nursing home) based upon an omission, and that the liability for an omission can only arise when there is a legal duty, but almost implies that only contactual duties are sufficient. It does so at paragraph 35 which says:
Importantly, even if the LINKS contract relating to Jared were to have
been renewed and to have been in force at the time of Jared's death,
we are not convinced that it would be the sole basis or even a
controlling factor in determining Defendant's legal responsibility
under the Act. Defendant's criminal liability must exist solely based
on an omission— a failure to act when she had a legal responsibility
to act. See Deborah A. Goodall, Penal Code Section 22.04: A Duty to
Care for the Elderly, 35 Baylor L.Rev. 589, 594 (1983) (stating that "
authorities have long agreed that before an omission can constitute an
offense[,] there must first be a duty to act" ); see also People v.
Beardsley, 150 Mich. 206, 113 N.W. 1128, 1129 (1907) (" The law
recognizes that under some circumstances the omission of a duty owed
by one individual to another, where such omission results in the death
of the one to whom the duty is owing, will make the other chargeable
with manslaughter. This rule of law is always based upon the
proposition that the duty neglected must be a legal duty, and not a
mere moral obligation. It must be a duty imposed by law or by
contract, and the omission to perform the duty must be the immediate
and direct cause of death." (citation omitted)).
But, this could be dicta because it was a case where any legal duty would arise under contract rather than for another reason, and as is the case in many smaller states, there is simply no case that has ever been decided in New Mexico which is squarely on point.
Under British criminal law, in similar circumstances, a homicide conviction was vacated:
R v Khan & Khan (1998) CLR 830, confirmed that there is no separate
category of manslaughter by omission unless the omission constitutes a
breach of duty to act. The defendants supplied a 15-year-old
prostitute with twice the amount of heroin likely to be taken by a
regular user. The defendants left her unconscious in the flat,
returning the next day to find that she had died of the overdose. Had
medical assistance been called, the girl would probably not have died.
The unlawful act was supplying the drug but the death was caused by
the quantity injected by the victim. The trial judge invited jury to
consider liability on the basis of the defendants' failure to summon
medical assistance. On appeal, the conviction was quashed because the
brothers had not accepted a duty to act before she took the heroin.
A dissertation on when criminal liability is imposed for omissions in Scottish law can be found here.
New Mexico, whose laws really should govern, has just two homicide statutes:
§ 30-2-1. Murder
A. Murder in the first degree is the killing of one human being by
another without lawful justification or excuse, by any of the means
with which death may be caused: (1) by any kind of willful, deliberate
and premeditated killing; (2) in the commission of or attempt to
commit any felony; or (3) by any act greatly dangerous to the lives of
others, indicating a depraved mind regardless of human life. Whoever
commits murder in the first degree is guilty of a capital felony.
B. Unless he is acting upon sufficient provocation, upon a sudden
quarrel or in the heat of passion, a person who kills another human
being without lawful justification or excuse commits murder in the
second degree if in performing the acts which cause the death he knows
that such acts create a strong probability of death or great bodily
harm to that individual or another. Murder in the second degree is a
lesser included offense of the crime of murder in the first degree.
Whoever commits murder in the second degree is guilty of a second
degree felony resulting in the death of a human being.
Walter doesn't qualify for any of these prongs of the statute.
§ 30-2-3. Manslaughter
Manslaughter is the unlawful killing of a human being without malice.
A. Voluntary manslaughter consists of manslaughter committed upon a
sudden quarrel or in the heat of passion.
Whoever commits voluntary manslaughter is guilty of a third degree
felony resulting in the death of a human being.
B. Involuntary manslaughter consists of manslaughter committed in the
commission of an unlawful act not amounting to felony, or in the
commission of a lawful act which might produce death in an unlawful
manner or without due caution and circumspection.
Whoever commits involuntary manslaughter is guilty of a fourth degree
Clearly, Walter also does not qualify as guilty of voluntary manslaughter. There is no quarrel or heat of passion.
So, either Walter is guilty in New Mexico of involuntary manslaughter, or he is not guilty of homicide at all.
New Mexico also has an unusual, and rather merciful "excusable homicide" provision at New Mexico Statutes § 30-2-5, that should also be considered:
Homicide is excusable in the following cases:
A. when committed by accident or misfortune in doing any lawful act,
by lawful means, with usual and ordinary caution and without any
unlawful intent; or
B. when committed by accident or misfortune in the heat of passion,
upon any sudden and sufficient provocation, or upon a sudden combat,
if no undue advantage is taken, nor any dangerous weapon used and the
killing is not done in a cruel or unusual manner.
Arguably, Walter falls under "excusable homicide" part A, as moving the pillow was a lawful act done without unlawful intent and that is what caused the death.