It is precisely because of this characteristic of the undead that both undead necromancers and the Wizard Wraiths, which are mages of various kinds transformed at the moment of death, are very eager to wage war.
And those undead creatures trapped by Zachary in the Cave were precisely in this awkward state where they could not die nor had any replenishment of death energy.
It is for this reason that Zachary, who had no understanding of undead creatures, came to realize the essence of these beings.
Although Zachary had been continuously feeding livestock, these animals, while having souls, possessed such low spiritual wisdom that the souls within them were too scarce. Though not completely devoid of them, it could only be said to be better than nothing.
However, Zachary was not a bloodthirsty person. Even though he would occasionally take action, he only killed the opponent's body, without intending to completely annihilate their soul.