Religion, specifically Christianity, plays an important part in Gothic Horror. Frankenstein is playing God, Dracula is repelled by crucifixes. But when it comes to D&D, it's no longer reasonable to assume that all players are coming from a Christian background, and thus will not understand the word "soul" in the same way. It means something quite different to a Buddhist, a Hindu, an Atheist, etc.
Even within Christianity, there are significant theological differences with regard to souls, some of which could have mechanical implications for D&D. Most obviously, do animals have souls?