All people hate (if we ignore some buddist monks for the convinience of introductory statement), but hater is a name someone gets for hating something (especially a book, musician, character,...) and than making sure everybody knows s/he does.
Your typical hater will not give 당신 an argumented reason, but will insist on the object of hatred is stupid/ugly/weird.
Haters are often trolls (or trolls are often haters. decide 의해 yourself) and will intentionaly post their opinion where it will couse the most aversion and/or provocation. On Fanpop, that is of course, on the spot where 팬 of the...