I think maybe you ought to talk to the police. Apostates have no more right than anyone else to harass and frighten people who are just going about their business. You might just go to the police station, say you understand this guy has the right to free speech, but he's starting to scare people and he's hinting around that he might do something violent. Ask their advice.
Also, read the book The Gift of Fear and act on some of the good advice in that book.