Dunno if it;s a sleeper or not. I wouldn't play it in an aggro deck, because I can damage my own minions. I would only play this if I don't have a board, but a 6/6 on turn 5 doesn't sound THAT impressive.
Acolyte of pain synergies, but will a control deck really run this minion just for this combo? I would prefer a removal or a taunt.
6/6 on empty board, which is probably going to be its saving grace. Might be played in Hunter to activate the bombs but that seems too slow and inconsistent, really. Might be useful for Warriors to make a board full of Grim Patrons.