One thing that tends to be overlooked is the beginning of Legion. You can take this however you want, but the implication is blatant. On the Broken Shore, the only reason the Horde survives the attack is because Sylvanas jumped into action and made a tough call. A tough call that put her val'kyr - her lifelines - in danger. If she were so selfish, evil, and didn't care about anyone but herself she wouldn't have gambled everything in that moment. Furthermore, when she is told she will be the next Warchief, she is visibly stunned and remains there for a seemingly long time until she is alone in Grommash Hold.
Using the Blight as a weapon can be all the deplorable things you want to call it, but it's little different than using mages on the battlefield. No one has ever had a problem with the idea of the humans and elves literally bringing a hellstorm down on the Amani trolls during the Troll Wars that melted them and instantly turned them into corpses but they have a problem with barrels of goop that turn people into corpses. It's really no different between the two. If you are against the use of Blight, then you are against the use of magic as a weapon in warfare. There can't be this double standard.
Sylvanas has an agenda. Sylvanas is a dark character. Sylvanas has done shady things. To say that Sylvanas cares absolutely nothing for the Horde or her people? No. That's an outright lie and is contradicted by the Edge of Night story where it states that the Forsaken would no longer be, "just arrows in her quiver."