Like everyone else has said, it's probably best to avoid being entangled in these imbroglios, and her boyfriend does sound like a potential threat, but you have to decide how much you like this girl. Is she worth it? If so, then it doesn't matter what her current boyfriend thinks. The only thing that matters is that you two both like eachother, and could possibly become passionate about eachother. That's the only thing. The way I see it is you have to keep your eyes on the prize. Her boyfriend is just an obstacle.

On the other hand, I can understand her boyfriend's situation as well. If a random guy was making out with my girlfriend, I would be infuriated; and I could safely say that if this guy and I were ever to meet, there's a possibility that the encounter may be detrimental to his physical health -- not something I'd do on purpose, but when you're enraged, as you would be if you were to meet the guy kissing your girlfriend, things happen when you don't mean to, and though I don't know too much in-depth about these mental disorders, I will assume that these disorders can make it even worse for you. I would probably be a little perplexed with the whole situation. The only thing I can suggest is getting to know this guy better -- not through concordant friendship, but by whatever underhanded information you can dig up about this guy. From reading other people's posts, it seems like this guy could possibly come after your life. Once again, is this girl worth it? If so, I would also suggest carrying a weapon. He may come back harder, but he may leave you alone.

Now, I'm certainly not advocating a violent confrontation, but it seems to be a possibility. If at all possible, see if you can tactfully talk your way out of it. It may be a good idea to let her simply break it off, but then again, that could take a while, and he may figure that you're the reason anyway, and come after you, or go after the girl. Really, it all boils down to one question: Is she worth the trouble? If so, go for it, and improvise from there. I would suggest being ready for the worst, though, just for keeping safe.