A lot of first year law students have a hard time with these concepts:

1) You have the right to use force when aiding a victim, provided that the force is what the victim would have been entitled to use themselves.

2) You are entitled to "self-defense" when you are not the instigator (or have clearly signaled retreat if you are the instigator); there is a threat of unlawful force; which is imminent and immediate. All three must exist otherwise, it's not "self defense" and if the aggressor is killed, the proper charge would be manslaughter. Aggravated manslaughter is usually tied to the use of a weapon.

3) Non-deadly force is justified where it appears necessary to avoid imminent injury or to retain property. It is the force which "reasonably appears necessary to protect oneself from the imminent use of unlawful force on yourself. There is no duty to retreat.

4) Deadly force is only justified to prevent death or serious bodily injury.

5) Criminal negligence, such as firing a weapon in a crowded area, removes the need to prove intent to kill/harm a bystander and would also be an element of manslaughter.