People saying Riley has not motive are missing the point. As far as we are aware, she didn't even know Pippen was at the tower, never mind in the guard room. A quick look at the rota would tell her Steven should be in there. Now she DOES have a motive to kill Steven - Lizzie.

I said this in the other thread (and this would only apply if she's the rat): She goes in there drunk, with a gun, expecting to find Steven, sees Pippen instead and realises he recognises her. She panics and shoots him.