I said I don't want to be involved(in cheating with a married man) and get threatened?

So a guy I was friendly with way back has tried initiating a ual relationship with me, but the problem is that he is married now. I declined saying I did not want to be involved (hurting his wife). He has come back that people like me who as he says expose "cheaters" end up 6 ft under. I never said I was going to go to his wife or anywhere else. All I said was that I didn't want to be any part of his betrayal to his wife. Is this so terribly wrong that he should resort to lines like curiousity killed the cat?