I'm worried about what my dad will do to my mum, and what I'll do to him?

I hated my dad. He treat me like shite when he lived here and controlled my mum and scared us all, he had an affair and moved out. He wanted my mum to abort me and said I ruined his life, and although he never hit me he was very verbally abusive and came close. He's moved in with a new girl who's nice and he invites me and my sister round for tea every fortnight(she's the only one he ever seemed to love) and I've started going to build a relationship but he never pays child support and now that my mum has finally moved on he treats her like crap. He texts her calling her a slutt and other things and threatened to kill himself if my mum didn't take him back and took advantage of her many times. Her new boyfriend is cool, I like him more than my dad and he has 3 girls who my sister likes so it's going well and we're moving in with him soon to save money because we're poor right now and I don't mind because I'll be at uni soon anyway, but my dad gets sulky at the mention of his name and when I did mention him he got angry at me and sulked with my sister. He was a crap dad and I've relied on my granddad who is amazing and I like having my dad on a leash like this, he can't get angry at me because he's scared I'll stop seeing him. But when he finds out we're moving in with my mum's boyfriend he'll flip! I've seen him threaten my mum with violence before and only stopped because my sister was getting upset. I'm worried he'll beat my mum up, he's that sort of person and what also worries me is what I'll do to him. He's a lazy fatso and outta shape, I've got 6 inches on him so if he ever started on her I'd kick his head off, which I don't want because he's my dad. I'm not angry or violent, but I wouldn't let him touch her, and since her bf boxes he'd have a go as well. I fell sorta sorry for him becasue his life is shite, but he abuses my mum and spends his child support money on weed and pills. I dont want him to hurt my mum nor do I want to see him hurt, I just want my mum to be happy with her bf and my dad happy with his gf.