Chapter 22 - "Our Doubts Are Traitors" - Part 1

[Kn1ckerb0cker33]

I might be wrong but didn't Pippen say "Oh, it's you" or something of the like. I also agree that the shooter was going in there intending to shoot someone, just not Pippen.

After more thinking, I'm starting to agree with the theory that Riley is the shooter and possibly the rat. Firstly she was drunk on the roof when Pippen came in, so there's a good chance she hasn't even seen him yet. I also think she was intending to shoot Steven originally, perhaps in revenge for getting Lizzie kicked out. I don't think this is something she would do normally, but drink combined with strong emotions can make people do stupid things. She grabs a gun, goes in expecting to find Steven (he was meant to be there), sees and recognises Pippen instead (possibly from some contact with the mallers, or if she is indeed the rat (something I doubt)), panicks and shoots him.

I seriously doubt that Riley is a rat for the Mallers since she has been there from the beginning and I cannot think of anything the Mallers could do/give to her in exchange for her services to betray the tower.
However, the possibility of her being drunk or angry or drunk and angry and shooting Pippin is a possibility, although not very likely. I still believe that whoever the rat or rats might be were making sure that their cover wasn't blown by Pippin identifying them.