I don't think they were even dead. He killed Helya to create a proto-Eyir; someone that can transform worthy female Vrykul into Valkyr. We see living Vrykul get transformed by Eyir during Legion (Hyrja). We don't know if Eyir sacrificed herself first to gain that power or if by that point Odyn knew how to empower an existing Valkyr to convert other mortals into one. Or maybe she was just another Titanic Watcher that was transformed just like Helya and the difference between them and other Val'kyr is the origin. Don't even know how he powers the entire process; it's not anima so is it Arcane? Is it Holy (Tyr used Holy magic after all)?
The Valarjar however ARE created from the dead with Odyn's Valkyr circumventing Kyrian Watchers (and Spirit Healers) to judge souls AND bear them to the Halls of Valor where Odyn Ascends them.