Spaz...
1) You are NOT the worst person in the world, believe me. I've known you for what, a year now? Therefore, I know how kind you are.

2) You did nothing wrong. I think you should stand up for yourself and say to your roommates just what you said to us: "how can she like him but I'm not allowed to?". What would have happened if your roommate found herself in that situation? Would she have not kissed him out of respect for you? And is that what you would have wanted? Nobody gets to be with the guy that they like? No, I think you should apologize for hurting her, one time only, and then leave it alone. Do not apologize to the other one -- she is sticking her nose into a situation she doesn't belong in anyway. If you want to date this guy, then go right ahead. Don't sneak around... be honest and direct with your roommates. They WILL get over it, especially if it turns into a relationship.

3) If these are true friends of yours, then they'll be happy for you. If it doesn't turn into a relationship because he's not all that you hoped he would be, then you'll have saved the other roommate some heartache. Either way, if they are true friends, then this will be a mere blip on the radar a month or two from now. Having had college roommates for 4 years myself, I can say that when we get together now, 15 years later, we barely remember who hooked up with who. We just know that we remained friends no matter what, and didn't freak out at each other for silly things like a guy one of us kissed once at the end of a night at the bar.