Skoll Shorties

She committed genocide and sparked a new world war leading to thousands of horde deaths, ultimately all to serve her own selfish agenda

If by "selfish agenda" you mean winning a war, then yea I guess you have a point. But I don't think we can judge her too harshly for winning at stuff.

No, I mean like causing thousands of deaths necessarily all in service of her own selfish agenda that only matters to her

Conquering death is absolutely a matter of great importance to any Forsaken. Not to mention, depending on the validity of the statement, "This world is a prison and I will set us all free", her efforts might be relevant to everyone on Azeroth.

Unleashing the entire rabid scourge on the rest of Azeroth, killing thousands of lives (children included) just to escape death and gain a shit ton of power isn't a proper excuse.

As far as I can tell, she didn't gain any power from shattering the veil. True she unleashed the scourge, which was being controlled by a guy who constantly threatened to unleash the scourge anyways.. But again, arrows in a quiver. Sylvanas will always sacrifice 49 if it saves 51

(Spoilers underway look away now.) Sylvanas gained the powers of the Helm by shattering it. She also gained powers by all deaths that came from the 4th War. Bolvar was constantly at battle with keeping the Scourge at bay & only released them recently on the Legion to help us.