Alice is visiting Bob, who is on life support in a hospital, when Mallory comes into the room and tries to disconnect Bob's life support with intent to kill Bob. Is Alice allowed to use force against Mallory to protect Bob? There is no euthanasia law or other law allowing Mallory to disconnect Bob's life support, and Bob is conscious and actively objects, but is not capable of stopping Mallory on his own.
It depends on Alice's belief about who Mallory is and what Mallory is doing and whether Alice's force was reasonable.
If Alice is aware that Mallory's action is lawful, then Alice cannot use force to protect Bob. Is Mallory a doctor who is lawfully withdrawing treatment? (You say there is "no law allowing Mallory to disconnect Bob's life support" but I'm not aware of a jurisdiction that requires indefinite treatment regardless of the circumstances.)
If Alice honestly believes Mallory is attempting to unlawfully kill Bob, Alice can use force to protect Bob. Is Bob on life support because of Mallory's previous attempt on his life; is Mallory a hitman, a vengeful spouse or some other person with no lawful reason to kill Bob?
Self-defence is available as a defence to crimes committed by use of force.
The basic principles of self-defence are set out in Palmer v R,  AC 814; approved in R v McInnes, 55 Cr App R 551:
"It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but only do, what is reasonably necessary."
The common law approach as expressed in Palmer v R is also relevant to the application of section 3 Criminal Law Act 1967:
"A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large." ...
In assessing the reasonableness of the force used, prosecutors should ask two questions:
- was the use of force necessary in the circumstances, i.e. Was there a need for any force at all?; and
- was the force used reasonable in the circumstances?
The courts have indicated that both questions are to be answered on the basis of the facts as the accused honestly believed them to be (R v Williams (G) 78 Cr App R 276), (R. v Oatbridge, 94 Cr App R 367).
To that extent it is a subjective test. There is, however, an objective element to the test. The jury must then go on to ask themselves whether, on the basis of the facts as the accused believed them to be, a reasonable person would regard the force used as reasonable or excessive.
It depends a tad in germany...
Mallory is staff ordered to shut off Life Support
There are cases where life support can be turned off. The two most relevant cases are:
- the conscious patient telling the doctor,
- or the unconscious patient having a Patientenverfügung, that demands no life support is performed.
The moment the patient says so or the properly documented paper is in the hands of the doctors for an unconscious patient, they have to end all life-prolonging measures, and shut off life support - in fact, they are not even allowed to start such measures once they are informed of the wish for it.
In a similar fashion, in the absence of such a document and with an unconscious patient, the next of kin might have the life support shut off at some point if they can make it clear that such treatment is against the patient's wishes.
In Germany, turning off the Life support is not (active) euthanasia (which is illegal) but Behandlungsabbruch (end of a medical procedure), which is defined as the not providing or ending of procedures that could prolong life. Behandlungsabbruch is not a crime, if demanded by the patient or the next of kin can make it clear that it would have been the wish of the patient. In fact, it becomes a crime to not turn off the machines the moment that the wish of the patient not to be kept alive is made clear, either by the patient, his Patientenverfügung, or verified by the next of kin.
Because Mallory is not committing any crime in case he is acting on the Patient's wish (expressed by the conscious patient, unconscious patient's Patientenverfügung or unconscious patient's next of kin in absence of a Patientenverfügung), Alice can't legally use any force against Mallory.
Mallory is just out there to murder
Simple case: yes, she may use a reasonable amount of force to stop Mallory. In fact, she is obligated to assist, as long as she is not in danger from that.
The United States has some very robust self-defense laws, which would make Alice's actions perfectly legal. Typically, U.S. self-defense/defense of other laws will cover the minimum amount of force required to prevent a crime from occurring, up to and including lethal force, though because of the U.S. federal nature different states have different laws. New York, for example, requires that the level of force used be proportional to the criminal threat, however, considering the crime is 1st degree murder, lethal force can be used to prevent this type of crime.
In the U.S., it's not uncommon for armed police to remain at hospitals, even outside the door of the patient, especially if they are to be taken into custody (either as an arrest or protective custody... Bob could be a criminal or a witness who a criminal would want to end, as Bob could provide evidence against the criminal targeting him.).
Globally, some of the deadliest serial killers are called "Angels of Mercy" (or "Angels of Death"), who are employees in medical professions or hospitals (doctor, nurses, MediTech's, etc.) who kill patients to end their suffering. For example, Charlse Cullen, a nurse in New Jersey and PA area hospitals, was arrested in 2003 and convicted of the murder of 29 individuals, the earliest proven one occurring in 1988. It suspected that he could be responsible for over 400 additional deaths in his 15 year span, but records are not available to prove conclusively. Compared to non-medical serial killers, Sam Little, who was arrested 2 years later and was active for 20 years longer, was convicted of killing 61 people and is suspected of killing 93. Ted Bundy was convicted on 20 counts of murder and may have killed more than 100 people. The most profolific non-medical serial killer in the world, Colombian Luis Garavito is suspected of killing over 300 people, and was convicted of 193. Angel of Mercy killers, on an average, have a higher suspected victim numbers than non-medical serial killers, as they tend to be active in hospitals and target victims they believe are in pain, seeing as putting them out of their misery (hence the name "Angel of Mercy"). Since their victims are often in serious danger, it's not immediately suspect that they take a sudden turn for the worse. Cullen the investigation that lead to Cullen's arrest was actually detected by a poison control worker that the hospital called, who noticed that chemicals turning up in patients who died in nurses had no business being in those patients based on the scripts.
(1) Wer bei Unglücksfällen oder gemeiner Gefahr oder Not nicht Hilfe leistet, obwohl dies erforderlich und ihm den Umständen nach zuzumuten, insbesondere ohne erhebliche eigene Gefahr und ohne Verletzung anderer wichtiger Pflichten möglich ist, wird mit Freiheitsstrafe bis zu einem Jahr oder mit Geldstrafe bestraft.
(Anybody who does not render assistance in cases of grave danger or an emergency, even though it is necessary and within reason, will be put in prison for up to one year or will be fined.)
In particular, nobody needs to put themselves in serious danger in order to help. Therefore, if Mallory is armed or is raving mad and smashing things left and right you don't have to put yourself in harm's way (but you should still seek help and alert the authorities).
This legal obligation to render assistance is peculiar to the law systems in the French civil law tradition. By contrast, in jurisdictions in the British common law tradition such an obligation usually does not exist for third parties unrelated to the incident; it only exists for individuals connected to one of the parties or bearing responsibility for the incident.
In the Netherlands you are always allowed to restrain someone and call for the police or authorities... and when is restraining someone violence? Bit of a grey area I think, a lot of ways to interpret it if it comes to a trial...
If it was one of my loved ones that was being unplugged, I would stop that person no matter what punishment followed - deal with that later 😊.