I've been trying think of anyway they can swing this so the writing isn't terrible. It just doesn't seem like Sylvannas and not because it's evil because it wasnt thought out. Before this Sylvannas has always seemed conniving and strategic but this doesn't seem to be her brand of evil. Even the fact she didn't use the plague seems strange.

My only hope ( and I know it's a pipedream) is that Aszhara somehow influenced it. Imagine if her warbringers shows her taking Sylvannas' form. We saw in Legion she could change her appearance. The grey area would be while it wasn't the real warchief, the Horde still followed in the brutal act.

But realistically they've just made Sylvannas a 1 dimensional villian for no one's excitement.