Calling someone a cheater, liar, or skank is not a nice thing to do. You usually have some remorse or, if not, you're mad at them and trying to throw a low blow.
In conversation yesterday, I heard this description of someone:
"She is a cheating, skanky whore."
Under normal circumstances, that might seem harsh. However, in this particular case, it was the truth. Not so much an insult as an observation.