That fact that he was in a gang and his past abuse does make him dangerous but he also could be really trying to better himself. Go with your gut feeling in this situation. Have you talked to him about this and the way you're feeling? Why does he feel like he'll end up a serial killer is he really that angry at someone. Who would he want to kill and why? You need to find out those questions. Honestly if someone I was dating told me that I'd be really freaked out because I would not want to be with someone who wants to hurt other people.