I'm going to date my abusive ex's best friend
Are you sure they aren't also abusive?
Yes, I know him better than anyone else
Dating people out of spite virtually never works out well. I'd stay far away from that person. They deserve better than these reasons.
I really don't care what you think. I am going to date him, kind of out of spite.
If he finds out he could be really hurt and then break up with you. You should want to have a relationship out of mutual respect.