I wrote him a letter explaining why the messages and threats aren’t at all helpful and how they frighten me, but he seems genuinely unable to understand.
That was a good step, it would have worked with most people. Yet it was necessary. Why? If things continue, then any legal recourse could require such a notice to have been sent first. One approach would be to then have a lawyer send a warning letter to cease his contact efforts or legal action would be taken. As Skip suggested, best to continue not responding, in other words, be boring since there are indications of more severe mental issues than typical BPD.
Unfortunately things have not really progressed in any positive direction since then.
Despite my efforts to de-escalate his hostility, he remains fixated on what has become a vendetta against me.
He continues to bombard me with aggressive or disturbed messages... He is infuriated all the more if I don’t reply, but almost any reply I do give him is angrily rejected or leads to fresh accusations of how he perceives I have wronged him.
I tried to engage him in some ordinary chit chat to try to lift his mood and suggested we could be friends, but he simply isn’t able to have a normal conversation anymore. He reverts to the same repetitive set of phrases about suicide etc as if it is now a compulsion.
Suicide is serious stuff. It is best for professionals (emergency responders or trained doctors) to handle it. Since he hasn't committed suicide in the many years you've known him, it is probably more likely that he is using such claims to pressure you into continuing contact, to your distress. Can you admit that really you can't "help" him?
Ponder this: By continuing to respond to his emails (or texts or calls) you are actually enabling him to continue contact. By continuing to respond, you might even be legally negating your request for him to stop contacting you.
Does it make sense - as prior efforts have failed - to send him one last and truly final communication informing him, again, to stop contacting you, instead contact professional help, and that you are henceforth blocking his emails, texts and calls?
Be forewarned he may try to use other emails or other phone numbers to undermine your stronger boundaries.
You deserve some relief, some peace in your life.
