Deadly Physical Force
This is where it gets complicated. A person may use deadly physical force to defend themselves against deadly physical force. The law requires a multi-level analysis when a person claims they are justified in killing someone. It depends on a few factors: 1) who was the initial aggressor, 2) killer's actual beliefs, 3) what a reasonable person would believe, and 4) if the person was not in their home, could they have retreated in complete safety.
Killing someone is not legally justified if the killer provokes the fight. A person cannot bait someone into a fight and then kill them and claim they were defending themselves. This is true even if the fight starts off as a fist fight.
Actual Beliefs - Subjective Analysis
This analysis takes into account what the killer actually believed. The person must have actually believed that the deceased was about to use deadly physical force before they killed him. It is not probably or maybe. There must be an actual belief.
What Would a Reasonable Person Believe - Objective Analysis
The defendant's beliefs alone are not enough. Jurors must also decide what a reasonable person would have believed in the defendant's position. If a reasonable person knew what the defendant knew and was in the same circumstances, would they have believed deadly physical force was about to be used on them.
This is a safeguard against people claiming justified killings in every case. New York courts have added the reasonableness requirement as an objective analysis to be used in analyzing a case.
The fact that the killer was mistaken does not matter if he honestly believed he was about to be killed and a reasonable person in his position would believe the same thing.
Duty to Retreat
A person is not required to retreat in their home. But, if you are out on the street and someone pulls a knife on you, you would not be justified in killing that person if you could have escaped safely. A person has a duty to retreat if they are able, as long as you are not in your own home. New York does not have a law that allows you to stand your ground wherever you are.
To have a valid claim of justification in New York, the person must not be the initial aggressor, must have had an actual belief that the person they killed was about to use deadly physical force upon them, a reasonable person would have believed the same thing, and they must have tried to retreat.
Florida recently passed a "stand your ground" law. Basically, it removes the duty to retreat in public. It allows a person to kill someone if they are lawfully in a public place and feel their life is in jeopardy, without trying to escape the situation first. From media accounts, it appears that the issue of an honest, mistaken belief and the "stand your ground" law are the biggest snags in this killing of Trayvon Martin. I've only read the same news as everyone else. I'm very interested to read all the witness statements if they become available.
A long post, but this video will summarize everything: