Why did you go to hate her?
Because she effectively locked down the entire Northern Eastern Kingdoms for the Horde single handedly and commands on of the Strongest armies of all the races?
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Fine Lets undo what Sylvanas has done.
Oh shit The Burning Crusade never happened because Sylvanas didn't appose the Dreadlords and the Scourge in the North alone and they went unchecked and enslaved Azeroth.
The Forsaken have done more to safeguard Azeroth then the vast majority of the races.
More than Darkspear, More than the current generation of orcs <whats left of them>, More than the Tauren, More than the Humans of stormwind who didn't even lift a finger in the Third War, and so on.
Blood Elves would they be alive let alone hold Silvermoon if the Forsaken never rose to power?
It's a certain fact that without the forsaken the Blood Elves would not only never of been added to the Horde. The would probably be extinct with no one opposing the Nathrezim brothers or Kel'Thuzads Scourge in the North the way the Forsaken had.
Finally lets stress it
Anti-hero
Anti-Hero
Anti-Hero
Blizzard is fond of Anti-Heroes who people or cultures / allies view as doing the wrong thing and they generally become the saviors of the race or many races
Zeratul / and the Dark Templar
Kerrigan / and the Swarm
Sylvanas / and the Forsaken
Illidan / and the Illidari
Evil? Evil is Gul'dan if you're using a measuring stick. Sylvanas? Not even close.