I think it depends on how close you are with his family. If they talk to you, and ask why you are no longer with him, I'd tell them the truth. But if your only contact with them was always with him, then if you reach out to them now, chances are they will just see you as some crazy ex (which you are NOT!).
If his actions involved something illegal, than that's another story. I'd tell them.
If it's just him being a shitty BF, with zero boundries, unfortunately, that's his right. And I'd walk away.
I was in a similar situation with a police sgt. Not only was he doing illegal things with his job, he was a serious psycho in his personal life, to include hurting his children. I was able to pass on info to the law, regarding his child abuse, but the rest didn't matter. No matter what I said, I was looked at as the crazy ex. He was a cop, and very good at knowing how to manipulate the story. It wasn't until his son's school sent his son by ambulance to the hospital, for being beat by the man, that they came back to ask me questions and take me seriously. Unfortunately, again, he got off because he knew how to word things. Today, he's married to a woman who had children of her own. He met her, and they got married all in less than a year. I was torn as to letting her know what whe was getting into. But, I walked away. I knew she wouldn't listen, and she was going to have to live it for herself.
It's a tough choice. I think you need to think about yourself with this one, and ask yourself if it will help. If not, just walk away.