It may be that it's not a criteria for not hiring them, but never-the-less something you want to keep an eye one once you do. And perhaps something you want to ask them about at interview.

I dunno, it takes a lot more than a fistfight to get put in prison for violent offense. Sure people can change but only a solid work history or trusted recommendation can give you any real confidence. You may have to fire this person, or they may have to report to an abusive manage for some period of time. Some offenses are difficult to walk away from untainted.

Conversely, someone who was already willing to violently attack another person is probably much more likely to do it again?