I'm mad at my best friend for dating the girl he knew I liked
And that made you mad at your best friend?
I just can't believe he can be that self-centered and insensitive
I understand, it's normal to be jealous when someone has something that you've wanted. You should tell him that he hurt your feelings. She might not have been interested in you I think you should consider that.
Nah, he's a douche bag and I don't want to forgive his douchy self.
Just because you feel hurt doesn't mean you should hurt him. What if they girl didn't want to date you, don't you think she can choose who to date?