You should've broken up when you suspected him of cheating- especially after looking through his phone. Trust is dead, move on.

You should be calling the police about the death threats, you should NEVER have been trying to play private detective. You should block all of their phone numbers, including your boyfriend. See your way out of the whole messy situation because you don't know these people, and all they know about you is what your boyfriend tells them- which clearly isn't good because you're getting death threats. Jokes are supposed to be funny- death threats aren't funny.

If your boyfriend told his friends you were the greatest, you probably wouldn't be getting those types of calls.

Yes, breakup with your boyfriend. You should want to be as far removed from people who do these types of things as possible. I also recommend saying less about why you're breaking up- just "you know, I don't think this relationship is a good match so we're breaking up- but I wish you well." and let it go. The busier he is with a new girlfriend- the less his friends will bother you.