In most jurisdictions, defense of self or of others is an affirmative defense to criminal charges for an act of violence. It acts to provide complete justification when the degree of violence used is comparable or proportionate to the threat faced, so deadly force would only be excused in situations of "extreme" danger. Justification for self-defense usually cannot be applied to actions committed after a criminal act has taken place. A rape victim who, after the rape is committed and the rapist leaves, subsequently finds and shoots the rapist, is not entitled to claim self-defense. Most other victims of assaultive offenses are similarly not entitled to this defense if they act in revenge. In all but one U.S. jurisdiction, using deadly force against a robber or burglar who is attempting to escape with property is likewise not justifiable (Texas is the only exception and holds the defendant to a high burden of proof that the action was the only means available to recover the property without a serious risk of death or serious injury).
Defense of others
The rules are the same when force is used to protect another from danger. Generally, the defendant must have a reasonable belief that the third party is in a position where they would have the right of self defense. For example, a person who unknowingly chances upon two actors practicing a fight would be able to defend their restraint of the one that appeared to be the aggressor. Most courts have ruled that such a defense cannot be used to protect friends or family members who have engaged in an illegal fight. Likewise, one cannot use this to aid a criminal.