All things considered and starting my 3rd listen from the start...
I know I'm going out on a limb here... but I'm linking all the foul happenings in the tower to one person... The stealing of the food and ammo, ratting out to the mallers, and the shooting of Pippin and Hope...

I think it's Pegs because she's mental... she didn't have an alibi for when the stuff was stolen (she said she was on the roof)... the party was HER idea... and Pegs and Kelly was the only main characters in the tower present for all 3 events... and like I said in my post about Pegs being mental... it would make a great twist and awesome drama between her and Michael!!!
PEGS I'm Calling You OUT!!!! and your "Mr Wiskers" voice isn't going to save you either!