Eh idk I get the impression most Voidsent are mindless and so incapable of premeditated evil deeds. The most evil thing about using them (assuming good intentions) would be forcing a creature to unwillingly serve you, and Voidsent used to be people so it's a bit of a dark grey area. (Edit: That's how it has been done in the past at least, there could be some pact made via magical means where we feed them the aether of our enemies in return for their service.)
Corpses used to be people too and using them is a no-no but Voidsent are what they are already and unlike corpses they'll kill people on their own so uh it could make for some interesting story if nothing else.
Ah using Eden on the 13th is an interesting thought. Could also be a reason we gain access to Void Magic if we're intent on going there. Not sure we would though since there's nothing left to save and even the Ascians can't use it.
