So even if her actions seem evil to everyone else (for now), it doesn't make her evil without knowing her true intent.
I don't get the feeling she is toying with lives/souls for fun... she is not that far gone is she?
A spider eats as many flies as it needs to survive. It doesn't make the spider evil. Even if Sylvanas does only care about her self it doesnt make her evil. Consuming lives/souls of others to ensure her own, imo she is just acting like that spider.
Also it might seem so at first to others but Sylvanas doesn't only care about her own survival and freedom. If that was really the case why would she yell that line "I will set us all free!" (in the reckoning trailer).
I think we are missing a bigger picture and with it her intent behind her actions. It might have something to do with the Light and Void being afraid or weak vs. the cosmic power of death and Sylvanas is collecting it with all her power.