Guys...? opinions pleasee why would you ever do this to a girl?!

well a couple of months ago I madeout for the first time with a guy I really liked. we never dated but kissed and acted like we were dating for about 3 weeks. but then his best friend started liking me and got mad at him for kissing me. the guy started blaming me saying all of it was my fault.
I've moved on now to a guy I really like but after a few weeks of dating him the guy I used to like asked me if I would cheat on my boyfriend with him and I said no. he seemed fine with that and we were friends but he kept telling me he loved me and I finally said stop.
it seems like he hates me now and he won't talk/look at me. he is telling my boyfriend I like other guys and is trying to ruin my relationship. why is he doing this? what's his problem?