I agree with the boyfriend for leaving. I'm assuming the boyfriend doesn't have kids so coming into a situation like that is extremely difficult especially if the parents aren't on good terms. OP should get her shit straightened out with her ex and everything before pursuing a relationship.

I have personally experienced the horrors of a past dysfunctional relationship seeping into the a new relationship. I've been threatened by an ex of an SO before. 

For anyone reading this, if you are seeing someone that has a violent ex, think twice. You do not want to get caught up in a shit storm of legal issues.