Why would he be nice to me after finding out I dislike/hate him?

there is a guy I work with I dislikew, I'm even being transferred because of this.Anyway- when we first met we hooked up pretty fast, things didn't work out as friends and I kind of ended the friendship but trying to be nice about it (he really hurt me and was being a d*ck)- however he acted like he couldn't care less

.He has been all over the new girl and rubbing it in every ones face that they re having a "thing" even though she was still engaged to someone else. (before he was still trying to flirt with me and touch me etc at work)

Time has gone on, and I am always civil to him, he sometimes tried chatting to me I'd be polite but generally avoid him unless it was work related.I had told a couple of people at work I hated him, but I didn't want anyone to say anything as it would make things awkward before I left.A few weeks ago, he was asking me about my art course and stuff. I was complaining to my supervisor that I had to stay till 8.30 when I was tired and had college and work tomorrow, then this guy said "be right back" and he went to the supervisor and organised it so he would stay till 8.30 instead of me, I said thanks to him was he sure he said yer.Then later he suddenly said "actually I need to talk to you--" I said why? then he said " I don't want you to pretend you like me. I'm not deaf, and I'm not blind. So seriously if you have something to tell me or talk to me about please do" I said nothing and some other guy interupted to ask something, after hed gone, he asked me again "so.. you've got nothing to tell me?"I just didn't say anything.... it was just awkward, we were just talking civilly for the rest of the evening.

Since he has been acting really on/off sometimes it seems like he is ignoring me other times he tries to be friendly again and help me out, or make jokes with me, why .... why would he do this if he knows/thinks I probably hate him?

p.s he also has told everyone at work including the new girl he is sleeping with, all about what happened between us, and personal things about me, and trying to make it seem like im jealous :/