Sylvanas gave specific orders to Nathanos to do these things as part of terror tactics. Same way she gave specific orders to him during the war campaign quests.
She is not being all kind and offers those NElves a “second chance”, they are raised as a terror weapon and are being manipulated into doing what bests suits her ends. Or some could be willing participants for their own reasons.
p.s. Lor’themar threatened to kill her if she raised any Blood Elves…
Honestly? It might stop 'em from getting ressed as Forsaken. What’s the point in spending magical energy on someone you know’s gonna just tell you to bugger off after all?
Abominations appear to be entirely new intelligences, so not really infringing on any free will of the past people is it? Unless you’re proposing that being created at all is violation of free will but that turns into a “I NEVER ASKED TO BE BORN, BANSHEE MOM” sort of thing.
I mean the abomination itself is a Scourge invention.
As undead, they still need souls to animate them, right? Those souls have to come from somewhere, and it’s not very nice to just take them without permission.
Isn’t Thaddius constructed in a similar way to most abominations, as in they take a bunch of corpses and just put them together? Because Thaddius is basically described as a prison for a bunch of souls of dead women and children.