Self-Defense
To succeed, a defendant arguing self defense must show that the killing resulted from a reasonable use of force to resist a reasonable fear of death or bodily harm. The defendant cannot have instigated the threatening situation. The degree of force used in self-defense must be proportional to the threat perceived, and the threat perceived must be something that would place a reasonable person in fear of death or great bodily harm. Mere words or insults do not suffice.
The defendant's reaction to the threat cannot take place after the threat of death or bodily harm has passed. Many states require that the defendant attempt to retreat or avoid danger if possible before resorting to the use of deadly force.
For example, if someone incapacitates a mugger with pepper spray, he or she may need to attempt to flee to safety instead of taking out a pistol and shooting the mugger. States differ in the degree to which they require an attempt to retreat if the threat they face occurs in the defender's home