“Hostile work environment” is one of the most frequently misunderstood terms regarding what’s illegal in the workplace. Discrimination based on race and sex (and some other things, but let’s stick to these for the example) are illegal.
One of the ways such discrimination can manifest itself is called “hostile work environment.” That means the environment is hostile to a person or persons based on their race or sex. The original precedent comes from women who claimed sex discrimination based on a pervasive environment of girlie mags, dirty jokes, suggestive comments. This was a breakthrough for women, since they didn’t have to prove something concrete like not getting a specific promotion because they were women, but rather just that the environment overall was hostile toward women. A similar situation could be an African-American employee whose coworkers made racist jokes, hung nooses around the office, etc. And yes, men and whites have the same legal protection in reverse, but the hostility needs to be pervasive and based on race or sex.
Where people get confused is that this does not mean there is a law against someone being just nasty and hostile to you at work. It may be against company policy. It may be against the law if it includes a physical threat, in which case as I said calling the police is not out of line. But there is no law that says the employer has to stop this. The law is against discrimination, and the hostility needs to be descriminatory against a protected characteristic to be illegal.
I hope that clears up the US meaning of hostile work environment. Some other countries (UK, some European countries, maybe Canada) are a bit more progressive in specific anti-bullying laws, although I don’t know the specifics.
Now, there would be good reason to fire an employee who makes a threat to kill another employee, unless it was in the most obvious of jest. First, common sense. Keeping the threatening employee will likely lead to turnover and loss of productivity. Any customer who sees that kind of behavior will be less than impressed. And so on. Also, there is a legal angle, in that if the threat is in fact carried out, and the employer was aware of the threat but didn’t act, the company is at risk of being sued for damages. From my reading of the OPs situation, managment was not made aware of the threat, and therefore couldn’t have been expected to act. Making management aware of inappropriate comments like “you ratted me out” or “OK with you if I work overtime, hee hee?” doesn’t, IMHO, rise to the level of managment needing to fire someone. Also, the damages at this point are at the level of emotional pain & suffering by the OP. That is much less of a legal risk than a death threat being carried out.
If the OP wants to go the lawsuit route, document everything and bring everything to management’s attention, and document that you brought it to management’s attention, and that management didn’t act. Documenting what happened is only half the story, documenting that management knew and didn’t act is the other half. This is not legal advice, IANAL, but I hope it is a valuable counterpoint to assuming there is an easy lawsuit on the table. It’s possible a lawyer in the OP’s state could find an angle I have missed.