I'm slow to adopt new words, but I think "hater" is a useful addition to the language, and not simply a pejorative version of "critic." The difference is that a critic is what you're called as a byproduct of being critical in a particular instance, whereas being a hater is an ongoing state of mind. So for example it is reasonable to say "he was sometimes a critic of the administration's policy" but not to say "he was sometimes a hater of the administration's policy."
"Hater" is not a pejorative version of "critic" so much as a negative version of "fan." In fact the distinctive thing about a hater is how uncritical his responses are.