You are riding a public transit train and out of nowhere a man with blue jeans takes out a P99 semiautomatic pistol and takes the passengers hostage. Luckily you have a knife on you. But you don't know whether it is legal to kill a person under such circumstances. Hence the question: is it legal to kill the man who is threatening your life, and the lives of countless others? Context is California, USA.
I would just like to clarify, in addition to the other answers and what Dale M alluded to, one important detail: Unless you are carrying out the death penalty, no one under any circumstances is allowed to kill anyone else. What you are sometimes authorized to do, is to use deadly force.
There is an important distinction between the two. When using deadly force, you are using extreme force to stop someone doing something, which may result in getting that person killed, but killing isn't the point, stopping whatever he is doing is.
If instead of a knife you had a gun on the train, you shot the guy in the face, his crime spree came to and end, yet he was still alive but unconscious, and you decided to "finish the job" and shoot him some more, you'd be going to prison.
The language is important. Even if in self defense situations, if you say that you were shooting to kill, you're going to be in serious trouble, but if you say you were shooting to stop, you're in the clear.
According to California Penal Code Section 187
- (a) Murder is the unlawful killing of a human being, or a fetus, with malice aforethought.
Penal Code section 197 and 198:
Homicide is also justifiable when committed by any person in any of the following cases:
- When resisting any attempt to murder any person, or to commit a felony, or to do some great bodily injury upon any person; or,
- When committed in defense of habitation, property, or person, against one who manifestly intends or endeavors, by violence or surprise, to commit a felony, or against one who manifestly intends and endeavors, in a violent, riotous or tumultuous manner, to enter the habitation of another for the purpose of offering violence to any person therein; or,
- When committed in the lawful defense of such person, or of a wife or husband, parent, child, master, mistress, or servant of such person, when there is reasonable ground to apprehend a design to commit a felony or to do some great bodily injury, and imminent danger of such design being accomplished; but such person, or the person in whose behalf the defense was made, if he was the assailant or engaged in mutual combat, must really and in good faith have endeavored to decline any further struggle before the homicide was committed; or,
- When necessarily committed in attempting, by lawful ways and means, to apprehend any person for any felony committed, or in lawfully suppressing any riot, or in lawfully keeping and preserving the peace.
A bare fear of the commission of any of the offenses mentioned in subdivisions 2 and 3 of Section 197, to prevent which homicide may be lawfully committed, is not sufficient to justify it. But the circumstances must be sufficient to excite the fears of a reasonable person, and the party killing must have acted under the influence of such fears alone.
Defense of others: 197.1 talks about any person - this raises the concept of "defense of others." Generally speaking, one can use force to defend another person if that person could have legally used the same force. In the example you give, the person being held hostage could have used deadly force to defend themselves and, therefore, a third party can use deadly force defending that person.
Here is a video from a defense attorney and former California District Attorney giving a good lesson on California self defense laws: California self-defense laws - A former D.A. explains (YouTube).
I'm licensed to teach firearm law in Arizona and Louisiana. We never teach "shoot to kill" or tell people that they can kill someone. The legitimate purpose of self defense, using firearms or any other force, is to stop an imminent attack where the defender reasonably fears for their life or grave bodily injury. If someone happens to die as a result of a person using reasonable force in a self-defense situation then they may, indeed, be charged with murder. It will be a matter for a trier of fact, a jury or a judge, to determine if the use of force was reasonable and if, therefore, the level of force used was justifiable as an affirmative defense against the charge of murder.
If you have a reasonable belief you or someone else is in imminent danger you have the right, under California law, to defend yourself with "no more force than was necessary to stop the threat." Which means that yes, in that situation you would be acquitted of murder and assault charges.
However, if the knife falls afoul of local weapon laws, you could find yourself facing weapons charges.
(I am not an attorney so make sure you also consult an attorney for legal advice.)
The doctrine of self defence makes it legal to use reasonable force to defend yourself and others against the threat of unlawful and immediate violence.
There is no question there is a threat of violence, there is no question it is unlawful, it would be legal if it could be demonstrated the threat was immediate or that a reasonable person would believe it was and the force you used was reasonable also - using force to disarm and restrain the perpetrator is reasonable; continuing to use force after that is not.
There are two separate issues here. One that hasn't been mentioned is the danger you pose to the hostages. Unless you are capable of striking quickly, silently, and accurately to instantly kill or incapacitate, you run the risk of the gunman turning on you and/or the other hostages and opening fire. Your "intervention" could cause far more harm than good.
Of course, if he's already shooting people, or you are convinced he's about to shoot someone, the balance of risk changes dramatically. But in general it's best to let the pros handle the situation unless you really know what you're doing.
The second issue is, as has been mentioned, whether it's legal to kill someone once you've decided the lesser risk is an assault. Assuming your decision to attack is reasonable, it's legal to do as much harm as required to mitigate the threat. And the way you mitigate threat when faced with someone who can kill you is to shoot, stab or bludgeon with the absolute intent to kill. If the guy happens to survive, that's fine, but you don't ever take his health into consideration until after he's no longer a threat.
If you can easily get a headshot, take it. Now. Don't try to scare him. Don't try to impress him. Don't wave your weapon around and threaten him. Especially when he has a gun and you have a knife. Your goal is to incapacitate him. To best do that, you need to radically disrupt his central nervous system. The best way to do that is through lethal means. Barring that, severe blood loss will take him down, but it could literally take a few minutes. A few minutes of him shooting you and everyone around you.
So if you can't get an instant takedown, and you've started fighting, you keep fighting, with all your strength, until either A, he's not physically capable of harming you anymore, or B, everyone else has evacuated and you can safely run away. For point A, it could be because he's dead or unconscious, or it could be because you've managed to disarm him and you are capable of restraining him, or it could be because he's running away, etc. For B, safety is still priority. If the guy drops his gun, you don't give him the chance to retrieve it and shoot you in the back as you run away. Maybe you can get away before he gets the gun. Maybe you can't. It's a judgement call as to where that line is, but you don't stop attacking just because he's disarmed, especially if he's physically larger and stronger than you; you stop attacking when he's no longer an immediate, dangerous threat.
Now, there are many circumstances where you know you can't win. In that case, your attack is a distraction to give everyone else a chance to get away, then you disengage and hope for the best. Not because you want to not kill him, but because it's the safest option for you and those around you. In many cases, the safest option is to simply run like hell. Just like in the hostage situation, sometimes the best offense is letting the cops handle it.
Also, you may have access to non-lethal means of incapacitation. A Taser, used properly, is not just non-lethal, but probably far safer for the victims than trying to stab him with a knife. If you have a Taser and something like a rifle, there's a weighted risk here. The rifle is more likely to get a clean hit, from farther away, but the Taser does have the potential to keep him alive. It's another judgement call, but unless you're confident you can get close enough, and hit him where the leads will bite through, the rifle is the default choice.
He gave up his right to life the instant he tried to kill people. You don't give it back until you are confident that your right to life is safe, or there's another option that's safer than continuing the attack.