SPOILER!
Gerard had that Elvis-like sneer that always creeped me ut. He just looked so damned mean! He did what he did for hate. He was possessed by the spirit of Judah Zachary, a really bad dude, who possessed Gerard and made him to evil things.
Quentin actually was doing what he did because of someone he loved--Jamison. He had the wrong way of showing it, and went about it cruelly, but he was twisted in a human way. The way David and Amy communicated with him, via a disconnected telephone, was pretty damned frightening!
Love, Robin