despite what Varodoc might believe, that's not a fate "worse than death", it's simply....a death.
They just died immediately died in the Void by shock, and then their souls immediately went to the Shadowlands (okay to the Maw, but still they didn't stay in the Void for "an eternity of torment" or anything like that).
This because, as living beings, their souls are still connected to the Shadowlands, no matter on what plane of existence they die. Just like a Demon, no matter where he dies, will always return to the Twisting Nether.
So...yeah. It's a sneaky and cowardly way the Ren'dorei have to kill opponents, but the dead still go to the Shadowlands, Void didn't claim them at all.
until the Ren'dorei and Highborne can produce a magical weapon of mass-destruction that have the potential to wipe out thousands in an instant and destroy a whole city, I would assume...not...yet....