re: Need advice on handling workplace violence/threat/harassment.

I wouldn't quit. I'd consult a lawyer. If I could record myself while on the job I would. You're gathering information that may be used in a Court of Law, not just for HR purposes. I'd reach out to HR again and tell them that you are concerned about the working environment and your specific working situation. I would probably also mention concern about retaliation. If there is an obvious compromise (even if temporary) on where you sit I would ask that it be pursued. This way, you have evidence of your concern and also evidence they did nothing if something sinister goes down. If they won't respond to you, your lawyer might get their attention. I would do that at a last resort, however. Either way, I'd make that fricker and/or the company face the music without escalating the situation.

edit: And of course this does not constitute legal advice. ;-)