Doesn't make it magically ok. Alleria taps into the Void to combat the Legion. What will she think when she finds out Slyvanus' people accidentally raise mindless undead and then kill them all for a selfish purpose?
Which doesn't change the fact Forsaken wanted to create a plague to spread undeath, raising free will forsaken.
Obviously there is a difference being one is an accident and the other is not. Doesn't change the fact forsaken raise people at rest only to kill them after they become mindless. Alleria may use the void, but she does it for noble purposes. There is nothing noble about wanting to spread undeath.