How mean is too mean?

Okay. So. General fandom question.

I'm wondering: if you're writing/drawing/pondering/whatevering a canonically mean character, is there such a thing as too mean? Is there a point where the meanness or asshattery of a character gets to a level where you can no longer believe it? Do you lose sympathy for/interest in that character, or does the over the top, stone cold jerkface leave you dying to know what he/she is going to say/do next?

(This is marginally related to the chapter of Two if by Sea I'm working on, but mostly I'm just curious about other people's opinions.)

I've had a very busy day today, but I'm happy to report that I was able to find time to write! Yay!

Now I must away, and tend to the pot of chili that may or may not be bubbling over...

~later