It was once a word used to describe those who hate others for
irrational reasons, for the sake of hate itself.
Today, the word is mainly used by those who are hated on, even though they may be hated for perfectly rational reasons.
So we have
controversial figures like Kei$ha disparaging such "haters" because they point out her obvious musical, physical and metaphysical short comings, all of which are very much a matter of
scientific fact. These people are no more "haters" than they are "people with perfectly valid opinions".