Should I tell the university about this guy?

If you have a solid reason to think he is dangerous, then yes, you should tell the school. I'd start with the dean of students.

What is likely to happen is that the dean will report his name to a committee that keeps track of "students of concern" and they will look into his background and his situation and decide what, if anything, to do.

If you just didn't think he deserved to be at that college, I would tell you this isn't your business. But that doesn't appear to be what you are talking about.

You know him and I don't. You know his background, and I only know the brief bit you posted here. You have a sense of whether or not he is dangerous and I don't. So I'm going to have to defer to your judgment about whether this is something the school ought to be aware of. They're not going to just up and kick him out (unless he answered a question like "Have you ever been convicted of any felonies? If so, describe" dishonestly -- have you heard of the Gina Grant case?), but if it is your belief that he is dangerous enough that the school should be aware of it, I think you should act on that belief.

Even if you told the University, there is nothing they could do. Having a clean criminal record is not a prerequisite at most Universities.

Honestly, I think you should get to know the guy before you judge him. Maybe he just feels like he doesn't belong. I think you should knock and his door and introduce yourself. It might blossom into a an actual friendship if you're lucky.

I too feel its none of your business.