She hasn’t been written as a villain over 2 expansions. Some of her deeds are considered villainous, such as the burning of Teldrassil.
The thing everyone who loves to hate on her keeps overlooking is that every time she actually talks about her plans they include things such as “set us free,” and usually talks about the endgame.
Things to consider: if she is looking to conquer death and free everyone from its clutches then what she is doing she doesn’t consider it evil. She’s not killing people because she enjoys it, she’s doing it because she wants to gain power to overcome something that she feels looms over and threatens everyone: death. If she conquers it, and possibly even the Jailer, it’s possible she is planning on returning everyone and setting them free.
It f she is planning this then she sees herself as a savior and not a villain, and if those are her intentions and they do succeed then she is perhaps the good guy of the xpacs.
It’s also possible that she is doing it simply because she is evil and wants to rule over everything and everyone. She might be completely evil.
That’s the rub. We don’t know, and won’t know until Blizzard reveals it.
All I know is that all thru BfA she wasn’t spouting things like “I’ll rule them all,” or “I will have all the power.” She stated things like “we will be free from death,” and “I will set them free.” Her shout of “the Horde is NOTHING!” actually further solidifies she has bigger plans as in the grand scheme of things the Horde can’t stop death or their subjugation to it.
This is all written from someone who doesn’t even really like her character. I just try to actually decipher what Blizzard is hinting at and leading us towards rather then “Sylvanas is evil because I can’t stand her.”