Genn makes a mistake trusting a sorceror who thought he had more of a handle on the Worgen then he actually did. Why are we blaming Genn for Arugal’s betrayal? That doesn’t justify what Sylvanas did at all. Worgen or no Worgen, Sylvanas was gonna invade. Ostensibly because Garrosh told her to, but really because at that point she was trying to raise as many new Forsaken as possible. Even if we’re blaming both parties, it’s like 10% Genn, 40% Arugal, and 50% Sylvanas.
So yeah, the Forsaken are responsible for the blight bombing and working people to death in mines while a werewolf apocalypse was happening. At least Sylvanas is.