She herself said all living are a threat to the forsaken.
If she controls the Valkyr do you really think she would allow anyone that isn't 100% on her side rezed? She fears death more than anything.
If everyone and everything is undead it means no more kids meaning your army is finite.
Most Alliance players and the writers don't justify every single bad thing they have done because the writers typically have every bad thing they done be by an individual or a small group of individuals while the leaders and majority of the races stay "clean". And the Offender is punished/condemned.
Horde on the other hand always has its atrocities done by its leaders and large groups of its people.