1. Home invasion by a stranger. 

2. Attempting to mug, rape or kidnap (bascially all of the more violent criminal acts. Plain ol' fistfighting is obviously excluded from that) you or another person regardless if the attacker has obvious weapon or not. 

3. Pulls out a lethal weapon with the intent to use it in a criminal act or threaten someone with it. 

And while I'm on the subject, your stated position sounds to me like it's dangerously close to the attitude of this guy, who deserves every bit of the prison sentence he is serving. 
https://en.wikipedia.org/wiki/Byron_David_Smith_killings

In my opinion a person is alowed to defend him/herself with lethal force if that person is in inniment danger. If someone points a gun towards my family, I as sure as hell would not take any chances. A criminal takes a certain risk with his profession. If that results in his death so be it, no reason for the state to grieve over that and convict the victim of the crime that went on when he killed the criminal. The crime must leave the victim in a dangerous situation, like being raped, kidnapped, armed muggery etc.

This. If taking someone else's life is the only way to protect yourself or others from serious harm. I'm shocked by how many here decide to throw human rights out the window just because someone enters their house or tries to rob them.

I don't see how it's close to this guy. If I were inhis position would I have shot the two people who by the way yes we're scumbag trash not worth the air they were breathing maybe, probably. But what he did was obviously excessive. Like for instance if someone broke into my house right now if I had a gun on me yes I would shoot them if I felt threatened but if they saw me and put there hands up in the air would I brutally murder them, no. Rough'em up, you bet your ass I would. if I am very confident someone isn't armed then no lethal force is nessicary but if I walked up on a burglar and couldn't see his hands there is no way in hell in warning them before I either 1 shoot them in the leg of I'm armed or 2 "jump them".

OK.  The reason I thought it sounded dangerously close was that we were talking about using lethal force on people and if you say "these guys don't deserve to be alive" that doesn't seem too far away from "they should die" and in the context of the discussion we are having that would pretty much be "we should kill them".  Since you say you meant nothing of the sort, then no more need be said—but I think your rhetoric lends itself to that kind of (mis)understanding. 

1) The victim's life must be in severe danger. Or theat of loss of limb or severe impairment or disfigurement.  Threat of death is not the only justified use of deadly force.  If you come at me with a fork trying to stab me in the eye, or slash at me with a < 3" knife, you'll still be shot until you stop.

2) The attacker has already injured/killed someone. There is no way for a person in a self defense situation to know this or attempt to figure this out in the heat of a split second life or death threat, nor are past actions relevent to the current immediate threat.

3) The attacker posseses a weapon.  - A weapon is not necessary to constitute reasonable belief of threat of loss of life or grave bodily injury.  Somebody repeatedly stomping your head or smashing it against a wall or trying to choke you, or threatening to run you over with a vehicle for example.

4) The attacker has invaded private property and posseses a weapon.  Again, a weapon is not necessary, and persons who are lawfully on the property do not have the burden to determine the invader's intentions if they reasonably believe the invader poses an immediate threat at any point during the invasion.  

Legally, there are also many exceptions which presume that anybody using deadly force to prevent or stop certain crimes is AUTOMATICALLY presumed to be acting reasonably.  In Arizona for example, crimes in progress such as burglary, rape, kidnapping, car jacking, and arson of an occupied structure, are explictly enumated to be reasonble justification in threatening or using deadly force without any further justification required.

Equally important is the defense of a third party clause where all previous stated definitions justifying uses of deadly force also apply when the action is taken on behalf of another person who is reasonably believed to be in danger.

There is also a legal concept of escalation of force.  For example even if you can't use deadly force to protect property in your state, you can use physical non deadly force to obstruct or detain someone from leaving with your property, and if in the process they become violent, then you can lawfully respond to attackers unlawful escalation to deadly force to defend yourself under the normal justifications of using deadly force to protect yourself.

These are two legally seperate scenarios using lawful non deadly force to protect property, and deadly force to protect yourself when the treat of deadly force is made by the burgler in response to the physical force.  Example 1: you fist fight a guy trying to steal your stuff or obstruct the door way, he goes for a knife/gun and you shoot him = legal.  You used deadly force to protect yourself, and non deadly force to protect your property.  Example 2: You stand in the path of your car that is the only path available to stop someone from stealing your car or attempt to remove them from the vehicle and they put the car in gear and show intent to run you over = deadly force was just threatened or used against you = justified use deadly force in self defense.  In states that forbid use of deadly force to protect property, these two circumstances must remain clearly seperated and enumerable in court.

It's important to note that the person who is in the wrong to begin with is always the one forbidden to escalate legally.  This prevents "well you should have let him leave with your stuff, you provoked him to escalating and pulling out a weapon" - this doesn't fly in court.  Any case precident set otherwise would essentially undermine and completely undo the whole premise of law enforcement - eg: it's the polices' fault for chasing and making those bank robbers shoot back.  The instigator is ALWAYS in the wrong escalating and the defender is always legally protected when escalating to meet force with equal force and this applies to both law enforcement and civilians and is a very important cornerstone in law enforcement and use of force laws.

if someone tries to rob you, rape you or kill you, I would say you have the right to do something about it, if it turns out you have no other choice but to kill the other person, you shouldn't be punished

It is not justified if the attacker is fleeing or not being a threat to you while lying in pain on the floor or something similar.

It is justified if you killed him by accident while protecting your own life or someone else.
It is only justified if it is the only option you could do to defend yourself and the death of the attacker was not part of your will to kill him.

Obviously, it has to be analyzed from case to case.
If someone is walking towards you slowly with a knife, it is in my opinion not justified to just kill him. Because you have all the time to flee or while having a gun, to take your time and shoot him and make him incapable of hurting someone. If these options are not present at that given time, you can shoot him, but still your aim shouldn't be to kill him in the first place like most police officers do nowadays. They shoot to kill and not shoot to avoid more damage.

Again, you should never solve a situation by having the will to kill someone else in the first place. That's murder.

Oh and.. according to the thread title... murder is never justified. You should change it to kill, because to murder someone and to kill someone have 2 different intentions behind it and are not the same.

I completly agree with you. Everything else what has the intention to kill someone else in order to solve a situation is murder.