Self-defense laws can vary somewhat from one state to the next, but, on the whole, they provide that people can use only a reasonable amount of force to defend themselves. How much force is reasonable depends on the circumstances, particularly the amount of force the supposed victim is using or about to use against the defendant. A defendant who acts in self-defense, but who uses more force than is reasonable for self-protection, is typically guilty of a crime—anything from simple assault to murder, depending on how disproportionate the force is.
Example: David and Goliath get into an argument at a lodge meeting. Goliath raises his hand and opens his fist as if to slap David across the cheek with the palm of his hand. The lightening-quick David pulls out his gun and shoots Goliath in the chest. David’s self-defense claim will fail—not because he acted preemptively (see Can I claim self-defense if I hit someone first?), but because he used far more force than appropriate to defend against a slap of the face.
For more on self-defense law, including issues related to defending oneself with excessive force, see Imperfect Self-Defense. Also see "Stand Your Ground": New Trends in Self-Defense Law, which addresses complications that concepts like “stand your ground” and the "castle doctrine" pose.