Originally Posted by
Golden Yak
Yeah, it's the 'burn it all down' mentality - it's all got to go, and it doesn't matter how. The demons destructive nature just makes them more effective tools to exterminate all life. Helps that Sargeras considers the Void worse than anything the demons do, so his atrocities can just pile up while he justifies it by saying it's all in the name of stopping an enemy who's even worse.
I'm pretty sure Sargeras' plan is/was, when everything else is dead and only the Legion is left, that he'll turn on and destroy them as well, until it's just him.