Is there anything else besides his threat that's preventing you from leaving? Living situation? Conflicted feelings?
Either way i'd just leave him. Whether he means it or not, if you leave him it's true he could possibly do it, but it not happening i'd wager is the more likely outcome. Besides, what choice do you really have? Staying gaurentees your indefinite suffering, probably until someone ends the relationship. Who knows how long that will be? Leaving now is less suffering for you. Even if they were to go through with it, it was ultimately their decision to make, not yours. Im sure you don't want him going through with it, and they're the ones intending to pin blame and guilt on you, for a decision they're making. So who has the possibly bad intentions here? Not you, you'd just be doing whats best for your wellbeing. What he needs is proffesional help, something you can only steer him towards, and not force him to do, and I doubt mant proffesionals are equipped to be a lifeline for a patient, much less you. You can't help him unless he helps himself, and staying would only reinforce his behaviour. So you're arguably doing him a favour as well.
I had a friend of mine that dealt with 2 experiences similar to yours, one of the ex partners has a group of new friends. Who knows if he's changed. The other was an off and on relationship, everytime they'd make the same threat, and when it ended, they'd be coming back to make amends.. rinse and repeat
Is this the first time he's made such threats? Do you have any info on his past relationships? What do your friends think of him? Im asking those questions to gauge him and whether he's been consistently like that. If so, even more reason to leave him.
As to how to leave him.. well, here are some approaches you can take, it might be of use. You'll know better than any of us if there's a right course of action for yourself among these, since you know yourself and him better than any of us. If you do end up reading these, remember there's a community Q and A at the end.
https://www.google.com/url?sa=t&sour...lEo_2r&cf=1