There’s even a name for that hill!
It’s called imperialism. 
I’ve said it once, I’ll say it again:
When you begin an expansion with some gigantic, horrific act of genocide like the Burning of Teldrassil, you’re effectively giving the A-OK for the victim side (in this case, the Alliance) to commit whatever atrocities it wants to, and get away with them scott-free. As long as they don’t repeat the initial act of genocide, literally anything else is considered fair game.
That’s what is meant by "the Alliance’s moral absolutism. Now, certain people can be sick of hearing that word—I certainly am—but let’s not pretend as if Horde players are simply making this up.