Thirst for power and need to conquer all? She is only working on regaining Lordaeron, what is so wrong with that?
How about we kill some useless leaders, like the dwarven council, or that guy in Exodar, maybe the nelf in Darnassus. They don't have a role anyway beside being 'leader'.
She wants survival for her people, she doesnt force them to join the Forsaken. They merely get ressurrected as undead and get a choice; join or simply don't.
Who are the Alliance to 'purge' Lordaeron from it's own people? Sylvanas is merely leading them for their survival and regaining land. And in war, everything is permitted (the plague).