Slayer seems like a pretty twisted guy. Maybe he killed trollocs for fun. As for the grey man, I think it has to do with the Forsakens' personal agendas. Maybe one Forsaken wanted Nynaeve and Egwene out of the picture, and another one wanted to stop that Forsaken, so he hired Slayer to do it.
As for Perrin, Slayer was definetly not trying to help Perrin. He gave lots of stupid advice and went against Perrin on a lot of subjects. It's just Perrin thought Slayer was doing those things 'cause he was an idiot. Besides, he was only appearing to cooperate. In the World of Dreams, Slayer tried many times to trap Perrin and kill him.
Tungwene