The privilege of self defense is based in the idea that no one should have to idly stand by while suffering harm from another, and one need not wait for resort to the law for redress. The privilege exists in both torts and in criminal law.
While a person may take reasonable steps in preventing harm to himself, once the danger passes the privilege of self defense disappears (revenge does not qualify as self defense). The privilege also extends to those situations where there is a reasonable belief of impending danger – one does not need to wait for an actual blow to be struck. But, while a person need not display abnormal courage he also cannot plead abnormal timidity as an excuse for his actions – the apprehension of harm that incited action must be reasonable.
A person cannot use an excessive level of force and then claim self defense – the actions taken in self defense must approximately match the level of danger threatened. If a person does use excessive force while defending himself he is committing a tort as to that excess, and the two parties may each have tort claims against one another. Deadly force may be used to counter deadly force, but only as a last resort – if there is a means of escape, one must avail oneself of it, though the courts are divided on this point. The one exception to the escape clause is when a person is in her own home – deadly force can then be used even if there is a reasonable means of escape. Some courts have extended this rule to include one's yard, place of business, or automobile, but these extensions are strained and should not be relied upon.
If a person inflicts injury upon a third person while attempting to defend herself, the courts will typically not find a cause of action in the absence of negligence. Of course, if the action is intentional, the courts are much more likely to find a cause of action – a man will be held liable for trampling another while fleeing danger.