Originally Posted by
Lightsworn
Sylvanas has always been evil. Back in Classic, she committed revolting experiments on war prisoners. She allowed the Forsaken to build an internment camp upon the ruins of Hillsbrad where to experiment and torture humans (she only decreed the Warden stationed there a traitor after he started to experiment on other Forsaken). Reports from Battle for Azeroth say that Stillwater is still alive, so his death might have been retconned and Sylvanas could have always allowed Stillwater to continue his atrocities in the Sludge Fields.
Back in Classic, Thrall kept Sylvanas under control. Back in Cataclysm, Garrosh kept Sylvanas under control. But now Sylvanas is Warchief, she has the ultimate power, and no one can contain her anymore. This was bound to happen, anyone who didn't see this coming clearly didn't pay attention to Sylvanas' backstory.