To clarify further, generally past convictions are inadmissible, because you don't want to bias a jury about the facts of the specific case. "This guy murdered

To clarify further, generally past convictions are inadmissible, because you don't want to bias a jury about the facts of the specific case. "This guy murdered three other people!" is no proof that he murdered this particular person. Because of those rules, calling someone a "godfather" in court doesn't so much get you a prosecution as it gets you an objection and mistrial. So I used a lazy example to illustrate the point. But this brings up another key point -- most of criminal procedure can be summed up by "you can't use irrelevant statements to prejudice the hell out of the jury." All in all a pretty good rule.