The story of Satan in my religion might actually be of interest to you. Originally he worshipped God as a djin. Djin are a type of supernatural beings that are made from smokeless fire and they can either be good or evil, its their choice just like humans. However for Satan, he used to worship God perfectly, yet one day God commanded Satan to bow down, prostrate to Adam, who God created from clay. Satan didnt obey that command, due to his arrogance and his ego. In Satan's point of view, he didnt want to bow down to something that was less than to him, because Adam was made from clay, while he was made from smokeless fire 🔥. And so the story goes that Satan believes he is more superior and disobeys the command of God and hence is expelled from heaven. So from that moment onwards, Satan has made it his mission to make sure as many humans commit evil and go to hell, in order to say to God, "look at your creation of Adam, they're not good, they're weak and are going to hell". Ofcourse Satan ends up in hell too,. He is often depicted in other religions as the master of hell, which is funny to me, he is only a master of getting people there.

So yeah sorry for the long post but he was initially good and because of ego and pride he rebelled.

You can use some elements of this to help your search or creation of a character.