I've like this girl for a long time now, we were such good friends and I must've fallen in love with her, she's helped me with a lot of problems and I've helped her, I think she liked me but I noticed she started having feelings for one of my friends, I convinced my friend to go out with her so I can see her happy, they only lasted a month and my friend didn't even have the balls to break up with her so I had to tell her myself, I was the only idiot though, I'm asked her out three days after they broke up, HUGE MISTAKE, MY BIGGEST REGRET, she rejected me but yet our friendship only grew stronger, she even used to say I was her best friend
Then one day, I don't know why, she just starts ignoring me, I tried to ask her but every time I do she shrugs, iasked her friends, they all say "she has her reasons". I've been so depressed and my friend that went out with her keeps telling me, "let her go man, she's not worth it, she's a hoe" I told him that there are something's or somepeople worth fighting for, then out of nowhere he asks her out and she accepted
I don't know what to do but I've never been more pissed in my life, I can easily beat the life out of him, but if I do, the girl will only hate me and I don't want to hurt her like that
What should I do?

You gave him "permission" to date her before, what's the problem now?
Th girl obviously does not like you as you like her.
Leave her alone. Maybe she and your friend really like each other.
Get over it and move on.