Hi Laslo, as grateful as you feel to your grandparents, remember that most people leave the nest at some stage as a natural progression in life.
Your grandmother possibly stays with him because you're there to soften the blow, and he makes no real effort to change his ways because he knows she'll return. It may play itself out eventually when he injures himself enough to need care, but how long do you and your g'mother have?
It might be worth sorting out a plan with your g'mother to get her out of her abusive situation, and allowing you to leave.
It doesn't have to be done tomorrow, but the situation is causing you and your g'mother enormous stress, and coming up with a plan, maybe with the help of professionals like social & aged care will at least stop you leaving suddenly with all the fall-out that results.