It’s possible that the gentleman admitted to it on his own accord because his bad temper and tendency towards violence are common knowledge and he knows that potential prospects will eventually hear it from someone else, so he figures he might as well admit to this himself. It’s also possible that this tendency towards violence has resulted in him having problems with the law, which he knows people will also find out about eventually. If either of these is the case, that is more than enough reason not to consider him.