I know some of you know what kind of company I work for. I'm trying to be a bit less direct in this post, however, so I'm going to dance around the field a little.
My company is small. It's in a small-city area, it's not a major corporation, it's more community- based. We are not the smallest of our type, but we are
not the big leagues.
Co-worker has been here for a few years. Co-worker apparently took the job as a stop-gap and now wants, desperately, to get out. He thinks he's worthy of the big leagues (and apparently has indeed done some work for them ... but he wants a full-time job. Frankly, I don't think he's good enough.)
So he's been incessantly bad-mouthing the place, the boss, the community non-stop. He's tried to get a young co-worker to quit, telling her how she'll be "stuck here" in this miserable area, how he's meant for better things, how he's had multiple job offers but is waiting for the big leagues, who won't take him because he "works for this joke of a place," about how he's "not even putting this (place)on his resume because no one will take him seriously." He's also a martyr, complaining frequently about how much wonderful work he does (when he really doesn't).
Boss knows that Mr. Negative is negative, but not how negative ... or that it's sometimes directed at him. (Frankly, there are some valid reasons to be negative that have to do with the corporation that owns us., which I think is what Boss thinks it's directed at.) In fact, Boss became annoyed at another co-worker who reported some of Mr. Negative's words by telling him "stop trying to get Mr. Negative in trouble."
Mr. Negative has become SO vicious that I'm starting to hear it leak out in his dealing with the community. Which could, of course, harm the job that rest of us are trying to do.
Now, it's not perfect here. But some of us are here because we LIKE the community-based stuff. Personally, I wouldn't take the "big leagues" for a lot of money. We're trying to make a difference here, to do things that matter, even if in a local way. But Mr. Negative has made it clear we're all miserable failures unless we want what he wants (and doesn't have) and go out and get it. (Nevermind that I've been here over a decade and I'm much happier with my life than he obviously is.

)
This guy is giving me an ulcer. I'm very, very close to simply letting him have it one day.
How can I handle this in a professional manner? Is my only choice ignoring him? I don't feel like I can say a word to Boss. I feel like I need to do something, not just because of my own feelings, but because he's going to start affecting community perception of a very community-type business if he keeps this up!
(Note: I can't go to HR. They're likely to just call up Mr. Negative and say "Elephantschild was badmouthing you!!!)