Here we do not have your fancy gods, Angels, and Demons. We don't have your culture or beliefs as we do not live in the same world and reality. For we are and yet are not humans, but what truly is human? What if we humans are not what we think we are, yet something far more dangerous.
In my world we live along side with what we call Demons. They are always in some sort of animal like form. Some people keep them as pets, sell them on the market like the simple kitsune fox, and others treat them as family. Yet, there are those few demons that are at war with the power hungry tyrants. For if a human manages to kill a Demon, they gain their power and become an Angel, a being of power.
Only the desperate do this for there is a higher chance of getting yourself killed rather then to make it out alive. Which brings in the other two groups you do not want to be in if you fail. Become a Defile if your body denies the power, for you lose yourself and your body becomes corrupted. Become a Fallen if you become full of hate and consume too much power for you to control and you are forever cursed with a twisted form between a human and Angel. If any one were to find any of these two they are to report to the Guardians so their souls may be put to rest. For all that we know, they are always crying, but why do they cry? Are they in pain? Or is there more we have yet to understand.