"Hater" is an insult often used by people who are too emotionally attached to a celebrity to accept any criticism of that celebrity no matter how true the criticism is. It is also often used even if the "hater" qualifies their criticism with a positive statement that far outweighs any negative statement made. To call someone a "hater" is an implicit accusation of jealousy, bitterness, and (in some cases) racism. In situations where the "hater" is actually more financially, academically, or socially successful than the accuser, the irony of that fact will always fly over the accuser's head.
A Miami Heat fan: "Steve Nash is one of the best point guards to ever play the game of basketball, but I think that Shaquille O'Neal deserved the 2005 Most Valuable Player award more than Nash."
A Phoenix Suns fan: "You're just a hater".
Me: "Eminem is possibly the best white rapper to ever live, but he is not the greatest rapper of all time."
An Eminem fan: "Shut the #@!$ up, hater."