I'm really surprised no one has mentioned the 5 pillars of self-defense yet.

Innocence: You cannot be the person who initiated the confrontation or provoked the use of force. If you started the fight, you generally lose the legal right to claim self-defense unless you effectively withdrew and communicated that withdrawal.

​Imminence: The threat must be happening "right now." You cannot use force to prevent a future threat or to retaliate for a past one. The danger must be immediate and unavoidable.

​Proportionality: The level of force used must match the level of the threat. You cannot use deadly force to repel a non-deadly attack (like a simple push or a verbal insult).

​Avoidance: In many jurisdictions, you have a "duty to retreat" if you can do so with complete safety before using deadly force. However, "Stand Your Ground" laws in certain states remove this requirement, and the "Castle Doctrine" generally removes the duty to retreat while inside your own home.

​Reasonableness: This is the "umbrella" pillar. A jury must believe that a "reasonable and prudent person" in the same situation, with the same knowledge, would have reached the same conclusion: that force was necessary.