I cheated on my BF and I am worried to tell him. What should I do?

I made a stupid mistake the other weekend. I was out with the girls and I got drunk. I ended up in bed with a friend of mine. It was a stupid mistake and I have no excuse. We agreed between us that we should not tell my BF, but I have a feeling that these things kind of come out and the guilt is killing me. I haven't seen my BF since, because I am afraid that he might break up with me and that he might hurt the other guy. He has been going through a bit of a rough patch these few months and this just might be too much. A few years ago he involved with some bad stuff but fortunately got out of it. He is in college now and is doing great. But he is an exceptional MMA fighter and what is worse is that he knows how to beat some one up and either make it look like an accident or make sure that they never say anything. I've heard the rumors about him from some of his friends. He has never done anything like that during the time I've known him, nor has he even raised his voice against me. But still I don't want anyone to get hurt. So what should I do now? I love him and I know it was a mistake. I don't want to hurt him nor do I want the other guy to get hurt. What should I do?