In criminal trials, self defense may be used to argue that the defendant's use of violence or deadly force were necessary to protect himself or others from harm. The rationale behind this criminal defense is that people should have the right to protect themselves, their loved ones, and their fellow citizens from physical harm. This concept is used in tort law as well as criminal law.
Several factors go into the consideration of a self defense case, including the question of who the aggressor was, whether the defendant's belief that he or others were truly in danger was reasonable, and whether the level of force employed by the defendant was reasonable.
The defendant does not need to have been attacked first to warrant a self defense plea. The imminent threat of attack is oftentimes enough to excuse the defendant for striking first. However, this threat must be reasonable and real. If John remarks casually to Mark, "Some day I'm going to punch you," and Mark immediately decks John in the face, a plea of self defense will probably not hold, since Mark was the actual aggressor. On the other hand, if John moves towards Mark with his fist raised and shouts, "I'm going to punch you!" then Mark's violent response may be excused as self defense, since John was clearly acting as an aggressor.
An unreasonably violent response to a threat is unlikely to be excused as self defense. If Allen punches Ted hard in the stomach and Ted responds by knifing Allen in the head, then it's very unlikely self defense will excuse Ted's actions. On the other hand, if Allen pulls a knife and lunges, then Ted could reasonably say he was in fear of his life, and the use of deadly force may be excused as an act of self defense.