I think I saw a thread on Fattit about Steven and Justins behaviour, and the just of it was that Steven probably sexually abused Justin when he was young, and is deeply in the closet (there was also a theory that Steven was probably sexually abused by one of his mothers boyfriends, and then sexually abused Justin) . When I re-played this episode in my head the relationship between Justin and Steven just made so much fucking sense.  It makes me dry heave everyone I see him  now, knowing that he's maybe not just a fat whiney manipulative fuck, but an actual psychopath who would probably be more dangerous if he wasn't an immobile flesh mountain.