And now I'm curious if Nidhogg is sealed at the bottom of the Dungeon by six Spirits (or five now that the poor Spirit who got corrupted into that plant monster is dead).
Doesn't a greater population mean more deaths, in absolute numbers? Sure, his stupid plan might cause a burst of deaths, but in the long run, for sustained mortality, what he needs is simply a greater population.
There are more gods of death than just one, though. No matter what that dude promises, some other god could render it all null. But then again, a fundamentalist would believe a rock is cheese and shatter all of their teeth trying to eat it, smiling happily all along.
It's not just gods of death that sort out souls, but all the gods. Gods of Death just actually like to do so apparently. And Thanatos's contract most likely would have him seek out the specific soul as we know Gods can do that with Freya keeping a collection of souls she liked up there
I think it's more he is annoyed at how many mortals are alive now, causing him to be overworked with the increase in number of deaths he has to deal with. So he wants the dungeon freed, to return the surface back to a difficult place to live so the population goes back down to a more manageable level.