Undead don't necessarily have to be mindless. Not that it makes much of a difference given how soldiers fighting the demihumans attack you for no reason at all anyways, aside from the commander, for some reason? From what I recall (and I didn't get this from the game itself) the death rune was a chunk of the elden ring that got broken off, so now instead of dying everyone just carries on, and there was supposed to be some sort of cycle with the Erdtree absorbing souls for reincarnation or something? Honestly it doesn't make much sense, just like all the other souls games; there's no children anywhere, people don't seem to age, but old people still exist somehow, and people that are supposedly sentient, intelligent beings are just patrolling around killing everyone on sight for no apparent reason. Except merchants. Like 80% of people that aren't violent are merchants. It's all very gameplay oriented. Which I don't mind, but it does clash with the whole 'deep lore' thing.