You know, unless he's planning a dick move in the end? The player DK can just wear the Helm and command the scourge to fight against the Legion. They've expressly said that this time Legion means fucking business and it's now or never; so why hold back? Civilian casualties, well shit, people die in war. That's a given.
But I mean why risk losing against the Legion? Why would you not want the biggest, deadliest and most ruthless fighting force on Azeroth on your side against your biggest enemy ever?