Will you date your best friend's ultimate crush?

It's really sad because a situation happened with me like this one. I'm a guy and i had a friend who i was really good mates with. I had liked this girl for a while and i was building up the courage to ask her out. I told him this and said that i liked this girl. He'd never even seen her before but said he was proud of me for it. There was a massive movie thing where like all my friends from my year and the yar below us went to a movie (this was the year the girl was in). I went to buy some stuff with my best mate who was a girl and who my friend liked. I'm not sure what he did next but when i came back my friend and my crush were kissing right infront of me. I swore at him from across the room and walked off with my mate, the girl one in a storm. I'd never felt so betrayed. He tried to say sorry because he thought i was doing something with my best friend. He's a stupid kid and i told him nothing was happening between me and my other friend. No happy ending to this story.