Disclaimer: This is a mere story of fiction. I am not condemning the bible but merely putting my own imagination to work.
Prologue
Long ago God created Adam and Eve from nothingness. But what the bible does not say is that God created his first angel to serve beside him as his right hand man. This angel was soon better known as Lucifer, The Lord Of Evil. The story of the fall of Lucifer is different form what the Bible corresponds, you see Lucifer was never an angel full of sin. He was chosen for a greater calling.
A way to serve his Lord to the fullest. When God had let his humans set foot upon the earth, his people started committing great acts of sin. So great God himself could not even tolerate it even with his unending merciness. So Lucifer came up with a plan to punish the evil of the humans. He would send them into a plain where they will be tortured until their sin is burnt out of their soul. Sadly God could not bring himself go along with this plan.
So Lucifer taking great sorrow from his father's predicament ran from the heavens and into the earth. Where the Lucifer would use his godly power to take the souls of the wicked and punish them. Thus donning the moniker Satin, which means The Great Punisher in holy language. Once God found out of this he immediately went to go see that Lucifer was already at work. With this God knew the sacrifice Lucifer had taken was for the greater good. So accepting this he made ground rules.
The biggest and most important is angels could not attack demons and demons were not to attack angels nor could Lucifer or God smite their own. But there was one problem who would keep them in line because they could not. with this they both made ground and decided to find a human with a pure soul and make them half angel, half demon. With this they have bended the rules and allowed someone neutral to keep both factions in order.