Legion didnt happen the expansion after TBC. And even with that, Sylvanas' acts were framed as far, far more horrible than what Illidan did. Sylvanas burning down Teldrassil is essentially framed as the quintessential evil of Azeroth. It isnt exactly the kind of act that can simply be pushed under the rug because the writers want her to be the hero now. Pretty much the entirety of BfA was a long list of reasons why Sylvanas is the ultimate evil, now only eclipsed by pledging herself to a primordial being that wants to commit omnicide.