That's literally a war crime. "There's no civilians" is totally untrue and usually a basis to excuse war crimes. We call gassing people and killing civilians war crimes, for context as to what a war crime is. Did I say "war crimes" yet? I'm sure you know very well what a war crime is, right?
This is all 100% things that would get you on trial in the Hague, and if Garrosh counts as a war criminal in Azeroth when he didn't even touch chemical weaponry, then Sylvanas deffo counts as one. There's literally no argument against that. If you kill civilians, you're definitely a war criminal.
Both sides are supposed to have a lot of skeletons in their closets, but there's no denying, at all, based on the fact that, again, the Hague would define these as war crimes and the Azerothian understanding of war crimes is the same, which it seems to be going off Garrosh using a WMD being a war crime, that Sylvanas is a war criminal and consequently a villain.
Oh, and
also, your idea of taking all the civilians on Teldrassil hostage?
As per the United Nations on war crimes. Notice how this very list also includes biological experimentation, which Sylvanas oversaw and permitted, personally overseeing the new plague on two hostages, combining the two, and also extensive destruction and appropriation of property, which I'm fairly certain happened en masse at Teldrassil, given all of the civilian properties there, and also Southshore.
Here's a full list, most of which Sylvanas has done.
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