When your stated goal is to kill all life, you're a villain. It doesn't matter why you're engaging in omnicide, that simply provides rationale for your behavior, not justifies it. Sylvanas has actively been waging war with the intent to kill everyone she comes across since being revived by the val'kyr at Icecrown Citadel. Before then, she was still actively developing a plague equally lethal to the living and undead with full intent to use it as soon as possible. Since the plague was perfected, she's used it liberally, oftentimes choosing to simply plaguebomb a settlement into extinction to save time rather than engage in any other form of warfare.
As far back as Vanilla, the Forsaken were actively engaged in human experimentation to perfect, again, a plague explicitly designed to kill everything it comes in contact with. A plague she had no problem using against civilian populations in Southshore and Gilneas back in Cataclysm, when she was supposedly a "victim." Sylvanas was always Capital E Evil, the only difference is that the writing in BFA overall abandoned nuance and subtlety.
Her character wasn't assassinated, it was pushed into the spotlight and a bright neon sign was put up above her head reading "I KILL SHITLOADS OF PEOPLE FOR COMPLETELY SELFISH REASONS."
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The only people who don't see Illidan as a dangerously-unhinged lunatic, especially after Legion gave us quests showing his willingness to kill his allies and throw everyone around him to the wolves for the smallest scraps of power, whose obsession rules him beyond any hope of reason, were his fanboys who furiously beat themselves over "AYE YAM MUH SKARZ!"