In the setting I run, people aren't quite sure what causes undeath, only that it happens. Theosophists, clergymen, and arcane scholars all have their theories. It is true that undead creatures, such as zombies, wraiths, and vampires, have a dark vital energy that flows through their body, much like ki courses throughout the bodies of the living. But that is the only point where people seem to come to an agreement.
One theory is that the negative energy is an animating and corrupting force. Once that isn't sentient or necessarily evil. It just simply is a force, like the wind or gravity. Others apply more of a anthropomorphic, evil personality to this energy of oblivion that wishes to disrupt the natural order and unravel reality. It is true that undead descend into madness and violence the longer they are allowed to roam. Even intelligent undead become so disconnected from humanity and sociopathic that they too become uncaring and violent towards mortals. This theory is common among the clergy, and traditionalists. It plays off of a classic good vs evil bit and keeps many of the clergymen relevant.
Some philosophers apply a different explanation to undead and their relationship with dark energy. The negative energy is attracted to the dead, but the true animating force is the abandoned corpse. After a person dies, their soul leaves their body to move on, as if the corpse were an old child's toy. Abandoned after years of faithful duty to the soul, the corpse becomes rightfully angry at its role in the cycle of life and death. Fueled by a psychic echo of hate and betrayal, undead rebel against the gods that put them in this role. They are metaphysical anarchists that feed on souls out of vengeance and to fight the powers that be. As you can imagine, this line of thinking is very popular among necromancers.
A corollary of the second theory is that the bodies are naturally evil. The mortal vessels contain inside them the capacity to do terrible and horrible things, and these urges infect our souls when we are alive. Selfishness and sadism are the default settings of a mortal body, with acts of genuine kindness and empathy an anomaly of the soul. Once a corpse is free, it becomes uninhibited and using negative energy, acts on its primal desires against the world. This line of thinking explains the more sentient undead with corrupted souls, like vampires and liches. A more nihilistic view on undead, but one that has gained much more support after the last 80 years of endemic warfare in my setting.
With these and other ways of thinking about undead in my setting, it means undead are treated in different ways. Some are seen as monsters to be destroyed, much like normal, More often, there is a feeling of pity, of seeing a trapped and tortured body or soul and most clerics believe in giving them a final, peaceful rest. I want to get into this more in another blog post, but the thing I am thinking of most is that to defeat undead, I want it to take more than just turning and smashing with a mace. I haven't quite settled on what I want, but I think I have an idea. Until next time.