There certainly are cases in which the violent past of an attacker can be admitted in trial if that makes it more likely than not that a person's perception that he was about to be attacked was reasonable in the circumstances. The admissibility of that type of evidence depends a little bit on the nature of the case and on how relevant that information would be to supporting the claims of the accused. So there definitely is a mechanism for having evidence like that introduced, but it would depend on the facts of a particular case.
On March 6th, 2012. See this statement in context.