The five principles of self-defense are:
You must not have initiated the conflict with the victim (in most cases)
You must be threatened with immediate death or great bodily harm
Your use of force must be proportionate – usually, you cannot maintain self-defense if you shot an unarmed attacker, for example
Your use of force must be necessary to defend yourself (but see the “stand your ground” rule below
Your actions under the circumstances, taken as a whole, must have been “reasonable.”