"Oooo! If we do it with a Shadow Priest rather than (insert any number of fictional characters doing the same thing here) it'll be original!"
Heel-Face Turn
Slowly Slipping Into Evil
Evil All Along
Protagonist Journey To Villain
Good Girl Gone Bad
Blonde Guys Are Evil
That's the top 6 TvTropes that return from your bullshit suggestion. Each has hundreds, even thousands, of examples. You're not being interesting or engaging or a good writer when you pull out the same bullshit garbage that's been done to death. And your laughable deflection of someone referring to the shadow is Vengeful when your "Awesome Story Idea" is that he literally -goes vengeful- with shadow-magic and gets all anti-horde in your rote machismo storyline is pathetic.
And get your Evolutionary Psychology 101 out of here, Leotheras. You clearly have minimal understanding of narrative structures and delivery of emotion through character actions within said narrative.
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If it's my initial suggestion of going after Genn, then she'd get rebuked because Anduin couldn't hand over the king of one of the allied people. If it's the Peace Summit, Genn could fuck it up with an assassination attempt, an ultimatum, or straight out violence against Sylvanas.
Either of those situations could result in both Lordaeron and Teldrassil.