There are times when a person just cannot control themselves at the very moment, due to overwhelming grief or anger, and they do something illegal that they later come to regret. This is a legal excuse for individuals when committing an offense. However, there are no true excuses in the eyes of the law (other than real self defense), and would only serve to get the charge lowered.
There is a distinction in that the event must happen in the moment rather than after the fact. Planning something requires premeditation. For example, catching your wife in the act and then beating down the person would be valid.