I've been through a similar situation OP, but I got away. I'd thought over how to get away with killing him many times. Slit his throat while he sleeps, preferably away from your mother, take the corpse and a few valuable belongings and dump them somewhere they won't be found. Give yourself a few cuts, make yourself look like hell, go home, smash the lock, and call the cops. Say your house was broken into and that he put up a fight, so the took him. If you've done it right, no evidence points your way.