The name given to different grades of homicide offenses differs considerably from state to state. Colorado, whose penal code is based loosely on the Model Penal Code, does not have an offense called "involuntary manslaughter" and the definition of "involuntary manslaughter" would not be uniform from state to state that does use that term, although there would be some similarities.
Colorado has six homicide offenses that do not involve children (which is the offense of child abuse causing death) or especially vulnerable individuals like the elderly, or kidnapping where the victim is not found in which murder is conclusively presumed, which I list with the pertinent language (omitting, for example, language on what class of felony it is except when necessary to explain substantively different suboffenses):
§ 18-3-102. Murder in the first degree
(1) A person commits the crime of murder in the first degree if:
(a) After deliberation and with the intent to cause the death of a
person other than himself, he causes the death of that person or of
another person; or
(b) Acting either alone or with one or more persons, he or she commits
or attempts to commit arson, robbery, burglary, kidnapping, sexual
assault as prohibited by section 18-3-402, sexual assault in the first
or second degree as prohibited by section 18-3-402 or 18-3-403 as
those sections existed prior to July 1, 2000, or a class 3 felony for
sexual assault on a child as provided in section 18-3-405(2), or the
crime of escape as provided in section 18-8-208, and, in the course of
or in furtherance of the crime that he or she is committing or
attempting to commit, or of immediate flight therefrom, the death of a
person, other than one of the participants, is caused by anyone; or
(c) By perjury or subornation of perjury he procures the conviction
and execution of any innocent person; or
(d) Under circumstances evidencing an attitude of universal malice
manifesting extreme indifference to the value of human life generally,
he knowingly engages in conduct which creates a grave risk of death to
a person, or persons, other than himself, and thereby causes the death
of another; or
(e) He or she commits unlawful distribution, dispensation, or sale of
a controlled substance to a person under the age of eighteen years on
school grounds as provided in section 18-18-407(2), and the death of
such person is caused by the use of such controlled substance; or
(f) The person knowingly causes the death of a child who has not yet
attained twelve years of age and the person committing the offense is
one in a position of trust with respect to the victim.
(2) It is an affirmative defense to a charge of violating subsection
(b) of this section that the defendant:
(a) Was not the only participant in the underlying crime; and
(b) Did not commit the homicidal act or in any way solicit, request,
command, importune, cause, or aid the commission thereof; and
(c) Was not armed with a deadly weapon; and
(d) Had no reasonable ground to believe that any other participant was
armed with such a weapon, instrument, article, or substance; and
(e) Did not engage himself in or intend to engage in and had no
reasonable ground to believe that any other participant intended to
engage in conduct likely to result in death or serious bodily injury;
(f) Endeavored to disengage himself from the commission of the
underlying crime or flight therefrom immediately upon having
reasonable grounds to believe that another participant is armed with a
deadly weapon, instrument, article, or substance, or intended to
engage in conduct likely to result in death or serious bodily injury.
(1)(a) is murder with pre-mediation.
(1)(b) and (2) pertain to "felony-murder"
(1)(d) is called deliberate indifference murder.
§ 18-3-103. Murder in the second degree
(1) A person commits the crime of murder in the second degree if the
person knowingly causes the death of a person.
(2) Diminished responsibility due to self-induced intoxication is not
a defense to murder in the second degree.
(3) (a) Except as otherwise provided in paragraph (b) of this
subsection (3), murder in the second degree is a class 2 felony.
(b) Notwithstanding the provisions of paragraph (a) of this subsection
(3), murder in the second degree is a class 3 felony where the act
causing the death was performed upon a sudden heat of passion, caused
by a serious and highly provoking act of the intended victim,
affecting the defendant sufficiently to excite an irresistible passion
in a reasonable person; but, if between the provocation and the
killing there is an interval sufficient for the voice of reason and
humanity to be heard, the killing is a class 2 felony.
Subsection (3) is commonly known as "heat of passion" murder.
§ 18-3-104. Manslaughter
(1) A person commits the crime of manslaughter if:
(a) Such person recklessly causes the death of another person; or
(b) Such person intentionally causes or aids another person to commit
There is also an omitted manslaughter exception for terminating life support, DNR orders and hospice type treatment.
Manslaughter is a class 4 felony (there are six classes of felony with class 1 being first degree murder in most cases, class 2 being most cases of second degree murder, and class 5 and 6 reserved for less serious felony offenses).
§ 18-3-105. Criminally negligent homicide
Any person who causes the death of another person by conduct amounting
to criminal negligence commits criminally negligent homicide[.]
Criminal negligence is defined in the statute and caselaw as more serious than negligence in a civil action and is basically akin to the civil lawsuit notion of "gross negligence."
§ 18-3-106. Vehicular homicide
Suffice it to say that the murder weapon must be a vehicle.
§ 18-3-107. First degree murder of a peace officer, firefighter, or
emergency medical service provider
Suffice it to say that this is first degree murder as shown above when the victim is a peace officer, firefighter or EMS provider.
Clearly this individual wasn't a peace officer, firefights, or EMS provider, and wasn't killed with a vehicle, so the last two are out.
Clearly, none of the first degree murder categories applies. This wasn't premeditated, the homeowner wasn't trying to commit a felony, there was no perjury, the victim wasn't under the age of twelve, no drugs were involved, and this wasn't a case of deliberate indifference (e.g. shooting randomly into a crowd of people).
This was not a case of criminally negligence, as the thief's death wasn't simply due to a blameworthy accident.
There is a good argument that, but for self-defense, this would be a heat of passion second degree murder. If the circumstances in the struggle were that the homeowner intended to and did kill the thief, this would apply.
There is also an argument that this would be manslaughter, if the homeowner knew that there was a real chance that the thief would be killed but didn't know that this is what would happen and was only actually trying to knock him out, for example.
Implications Of Offense Of Conviction
This subtle distinction between an intent to kill and an intent to not kill with a recognition that death could result, matters quite a great deal.
The presumptive sentence for heat of passion murder is six to twenty-four years in prison (as determined by the judge) followed by five years of supervised release. This can be reduced by up to 25% for good behavior in prison, so the range is effectively 4.5 to 16 years in prison.
The presumptive sentence for manslaughter is two to six years in prison (as determined by the judge) followed by three years of supervised release. This can be reduced by up to 50% for good behavior in prison, so the range is effectively 1 to 3 years in prison.
Post-script On Self-Defense
Of course, this is almost surely actually a case where self-defense applies. But, as the comment to the other answer notes, analytically, that is a separate consideration and not part of the prima facie elements of the crime. If self-defense applies, the result is an acquittal of all charges.
In Colorado, the self-defense provision applicable in this fact pattern is as follows (omitting provisions inapplicable in this fact pattern):
§ 18-1-704.5. Use of deadly physical force against an intruder
(1) The general assembly hereby recognizes that the citizens of
Colorado have a right to expect absolute safety within their own
(2) Notwithstanding the provisions of section 18-1-704, any occupant
of a dwelling is justified in using any degree of physical force,
including deadly physical force, against another person when that
other person has made an unlawful entry into the dwelling, and when
the occupant has a reasonable belief that such other person has
committed a crime in the dwelling in addition to the uninvited entry,
or is committing or intends to commit a crime against a person or
property in addition to the uninvited entry, and when the occupant
reasonably believes that such other person might use any physical
force, no matter how slight, against any occupant.
(3) Any occupant of a dwelling using physical force, including deadly
physical force, in accordance with the provisions of subsection (2) of
this section shall be immune from criminal prosecution for the use of
(4) Any occupant of a dwelling using physical force, including deadly
physical force, in accordance with the provisions of subsection (2) of
this section shall be immune from any civil liability for injuries or
death resulting from the use of such force.
Also, it isn't impossible to imagine a fairly plausible scenario where self-defense isn't available.
For example, suppose it was Halloween or April Fool's Day, and that the apparent intruder was actually an actor hired by the wife as a practical joke. Suppose further that the wife was screaming at the husband while he was battling the intruder "Stop! That's just Fred, an actor, this is just a joke." Suppose that husband heard his wife, but had leapt into such a focused on survival mode adrenaline filled state that while he heard those words and even though a reasonably person in his circumstances would have understood those words, the true meaning of what his wife was saying to him just didn't sink in for him subjectively until it was too late.
Also, in this situation, the wife, might, in addition to the husband, be guilty of criminally negligent homicide, since from her perspective, in this scenario, the death was due to her blameworthy negligence.
In a case like that, an analysis of which underlying homicide offense applied would be relevant because self-defense would not be available. A self-defense defense would not be available because (1) the person was not unlawfully present in the home (he had wife's permission) and (2) he was not committing a crime or intending to do so (he was just acting), and (3) the husband had the information that a reasonable person would have needed to learn that fact before it was too late (because his wife told him the facts clearly even though he didn't process it properly until it was too late).