You could also just break the lease.
Tell the landlord, split the cost. I mean that is technically what would happen. Problem is if he doesn't pay his half you'd have to sue him for the remainder.
But let's not assume. Tell him you're leaving, then go; or if you're scared of him, just go and don't tell him. He'll either default on the lease or he'll find someone to share the flat or he'll find a way to pay it and continue to live there. I doubt he'll default, that would hurt his credit. You could just talk to the landlord honestly, telling him you need to move out and see what (if anything) the landlord will do to work with you.
I would find another rental situation before any legal issues come up with this one. If he's violent or abusive when drunk (or otherwise,) as a last resort you could have him removed by the police and get a restraining order.