Shades & Zombies are two half entities that were once whole separated; the spirit (Shade) and the physical body (Zombie).
Two Shades can make a whole, though they begin their lives separated and go mad the longer they are apart.
Shades were messed up from the start. All they had to do was exist and eventually merge with their other half. They instead became sentient, decide to go full violence, and murder everything including their halves. A number of them combine into giant monsters that may look like images of Hell. Even animals became embodiment of evil through that corruption.
Shades were supposed to stay sapient, being disembodied souls and all. It was the bodies that were supposed to stay mindless drones until they finished up the clean up duty. Something went wrong with the process which put shades at the risk of going mad at random and their bodies started becoming sapient on their own over time, which just made things more complicated.