For that reason, as "The forsaken" became to Horde allieds, everyone think they are pure evil, but in warcraft all are relative. For the Orcs, humans, dwarfs, Nightelves, etc are the worse in the world (the same for the allieds in reference with the horde) ...
The point is, Sylvanas suffer too much when Arthas twisted she in a Banshee, then when the Lich King lost his Control over the Scourge, Sylvanas took Varimatras as an allied and created the forsaken and make alliance with the Horde (the opposite of the Alliance that she was as a Sin'dorei).
P.S: Sorry my english is poor.