I didn't read all the posts, but don't go after him yourself. First sit down with your girlfriend and her parents, and even your parents if you have been physically threatened. Explain the entire thing, then call the police and file a report.
If you go after him yourself, you could end up in jail and he would win. If you didn't kill him he would then have nothing standing in his way between him and her. Do not handle this yourself. Forget what he said about going to the athorities, it's a bluff, call it. Phone records can be checked and tracked, even cell phone records. There is proof aside from your's and your girlfriends testimony.
Please, please, please, call the authorities now.
Oh, and start documenting everything, times, dates, what is said or happens. Documentation is really important, collect the evidence.
