The self-defense law generally excuses a person for assaulting or killing another person if the defendant's fear of harm was reasonable, as determined by a jury, and necessary to protect himself or herself.
The "perfect" right of self-defense may be used to excuse a killing altogether if four elements are determined to have existed by the jury. Those elements are:
1. The defendant believed it necessary to kill the deceased to save himself or herself from death or great bodily harm;
2. The defendant's belief was reasonable, according to the jury, for a person of ordinary mental stability;
3. The defendant did not aggressively and willingly enter into the fight without provocation; and
4. The defendant did not use more force than was necessary under the circumstances to protect himself or herself from death or great bodily harm.
If the jury finds all four elements existed, the defendant cannot be found guilty of first- or second-degree murder.
The defendant cannot use the right of self-defense if he or she was the aggressor — he or she voluntarily entered or provoked a fight by assaulting the victim, through language or by being mutually willing to fight, or leaves the fight and then returns to restart it. ...
Read the rest of the article here. With the exception of the "stand your ground" provisions, which tend to be state-specific, much of the overview would typically find application, with some local quirks and mostly minor differences, nationwide.