Errr, that's not what they actually think however. Even Genn doesn't hate all the Forsaken, and Anduin wanted peace with them enough to call for the Gathering. Nothing indicates that the Alliance's leaders view the Forsaken as abominations to be purged. The rank and file might, but said rank and file is also extremely obedient so that's not too much of a problem.
Lovely prediction. It also qualifies as the worst possible tactic. Just think about it:
Admiral: Your Majesty, our ships are in range of bombarding the township of Brill. Do we give the order?
Anduin: No, we should not risk it from here.
Admiral: Very well, do we deploy the artillery on land then?
Anduin: Of course not Admiral, we will charge into the town with our troops, bravely risking falling into an ambush, trap, or Horde counter offensive; and we will not use our superior artillery in any way doubtlessly.
Admiral: But, my King-
Anduin: Oh shut it, eye for an eye!
Of course, it can always be a massacre in Brill if the few Forsaken civilians decide to stay for the next bombardment to prove their martyrdom.
Undead are hardly humanoid, they are more like kobolds on the ''dignity and right to exist'' scale.
Some of you hordes are alright. Don't come to Undercity tomorrow.
I do think the Alliance will get its unbelievable massacre moment. Anduin will almost have a heart attack over it, but it's still going to be his fault. I think it's going to be Rogers, going ham on the Horde in most horrible way she possibly can. And it's Anduins fault for keeping her after 1) a massacre against surrendering orcs in Pandaria, bad enough to conjure up some sha and 2) disobeying his orders in Stormheim, getting lots of people killed and losing the airship. He can't punish Genn for that, but he could and should have punished Rogers. and 3) her screaming rage against Anduin personally in the book. A soldier this unstable should not be in a position of power anymore, unless you want some massacre to happen. Or unless you are so inexperienced and naive that you don't see it coming.
Apart from the massacre against the Forsaken civilians by the suicide-mission Nightelf druids ofc. I'm not saying it's not understandable! But it's still pure hatred and revenge.