I thought the ending was clear, he was a psychotic person suffering from his own delusions - he never killed anyone. The hookers, Paul Allen or anyone. He was just a horribly twisted person who imagined himself killing them, you can see in some scenes where he himself realize he's suffering from his own delusions. Like the time when he shoots the cop car and it explodes, or when he runs into the same apartment building twice. Even he starts doubting his own sanity, and his coworkers merely got used to him telling grimdark jokes and being mildly insane.