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 you 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 by 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 fans of the hated object gather.
A quick review of comments on Murtagh spot and you can find an ordinary hater-troll - hates and makes sure those who like Murtagh know s/he doesn't. And I am not talking about opening a debate here.
Example: This forum could be a constructive debate. I believe at least some Murtagh fans would be prepared to compare him with Eragon and a different opinion is always welcome. But already the discription of the forum speaks about "sucks" and "hotter"....