What does it mean when people want to hurt other people?

I have a friend who confided in me and he has told me that he thinks he is a bad person. I asked him why and he said "Because I have bad thoughts." I asked him what kind of thoughts and he answered with "Bad thoughts, sometimes I just want to hurt people just to do it. Not kill anyone but just hurt them. Am I a bad person?" I told him I don't think he is but then there are signs that he does like to do this. For example, the other day we were playing around and then he grabs my arms (I was straddling him, we were wrestling- combatives like things) and I motioned to bite him but I didn't. Anyway, he hit me in my lip while he was still holding my arms away from his face. I said Oww and he just looked at my face and he did it again but harder. Like he just wanted to make me hurt more than I am already. He told me I would never be one of the people he wants to hurt but he does it often. He said no one specific but just people. What is wrong with him?