So according to my sister and her psych courses she took in university, psychopaths feel no guilt or remorse, are extremely manipulative, and do things simply for the shock value. Some internet research has helped as well. They know that he is extremely manipulative and impulsive, so they all watch each other's backs around him.
The problem is, other than internet research I don't exactly know how a psychopath would act if he was in a situation like this. Would he milk it and try to scare the other characters as much as possible, would he try to trick them into doing something, or maybe he'd stick to one character only and do both of the above? Any other suggestions are welcome!
Edit: Thanks everyone for answering so quickly! I've really been able to develop this character more thanks to you guys. ^^
p.s. I've seen Dark Knight and read Death Note, and I should have thought of that before, because those are some pretty good examples XD