I really dont want it to be Riley but there is alot of evidence pointing her way until I heard Michael mention that the killer could be the traitor who had a change of heart and wanted to keep her identity secret and stay with the tower. once he said that Kelly came up immediately. at first kelly was doing all she could to disrupt the tower then after the war she did a complete 180 and became helpful so most likely she killed pippen after trying to convince him not to go through with the evil maller plan and being threatened by pippin to have her identity revealed.