Scratch some of what I just said. The shooter is Steven, it makes perfect sense. Him and Pippen never saw each other from what I can gather, so Pippen wouldn't have recognised him yet. Also, Steven was totally against going in with him, for 2 reasons - he didn't want Pippen to see him yet and it he would've had to have shot himself to avoid suspicion, if he was with Pippen when he got shot.

Looking back, it was Steven that basically forced Burt to kick Lizzie out. Clearly that was so that the mallers could get their hands on her. For me he's got to be the rat as well as the shooter.

(Hope this hasn't all already been said)