Girlfriends male friend fucked her and tried to cuck me

I’m still fucking pissed. This was about a month ago and I feel the need to get revenge but I can’t really do anything. Where do I start…

My girlfriend has this one guy friend that she usually hangs with like 1-2 times a week and yes I’ve had the conversation but she didn’t wanna give up her “friend”. I’ve always hated the guy cause she hangs out with him and he’s like just a stronger, taller better looking version of me.

Like a month ago he was at her apartment and I was coming to her place too and I didn’t know he was there. I have a key so I just walked in to them fucking fucking. She was like flat on the bed getting railed from behind. I started yelling and this fucking bastard tells me to “just relax and watch” like are you fucking serious. I wanted to kill them and die like I can’t explain it. It was a screaming match and dude SHE TELLS ME TO LEAVE. And the dickhead says to give it up “lil bro” cause he’s fucking me girl. He told her to make me stay and watch like it’s his fantasy or some shit. I wanted to leave but I didn’t want to jurs let them keep fucking so after like 10 minutes of yelling the guy fucking pushed me out and I decided to leave.

I don’t know what to do and I need to like get bigger and hurt this guy idk