im a horrible person

Okay, so theres this girl i go to school with. She's dating one of my friends. I've never really been tons of fond of her.. sorry that really doesn't make a lot of sense. She has hurt him a lot and I'm pretty sure she only talks to me because of the fact she's dating my best friend. So I'm not exactly nice to her behind her back. Okay, I'm horrible but.. And I feel really bad because she was just really nice to me, and I can't think of anything nice to say back. Something about her just doesn't set right with me. Now I feel bad because of the fact that I'm mean to her and she probably likes me.. God, What's wrong with me?