This guy is stupid. He could kill her several times and get away with it.

1. I assume that it's ok he drank the stuff. He really didn't see it coming.
2. First time he bashed her on her head with his leg. I would just beat her senseless with my legs (actually I would probably only stop when her head is a bloody mess). Then either dispose of the body or just call the cops. He has poison in his blood, her backpack and a silly log of their chat as a proof.
3. Second time when he escaped after the "operation". Take the phone, dial 911 and say "I have just killed a thief that got into my house", then go and whack her. Get rid of the evidence about the first kill.
4. Third time when he choked her with his legs. Don't let her go! Choke the fuck out of her sitting on top on her so you are not suffocating too. OK, she got away. Happens.
5. Fourth time on the roof. Slash her throat so she can't scream while dying and then maybe take a bullet. If there is no shot (good chance, it's hard to shoot with pain from sliced throat) - just lay on the roof and let Janelle go away after knocking for fifteen minutes. If there was a shot - fall back to p. 2 - tell the cop she was a crazy "surgeon". Both scenarios are way better than just hanging yourself.