Heresy: Mitchael and Lucifer are the same entity. The read dragon Michael slays isn't satan, but rather an alien entity (from outside God's creation) that the archangel killed in service to God before he went rogue.

Angels (and thus demons) don't have names as much as titles. Or if they do they are well hidden from mortals. They gain various titles that reflect their job (for angels) or (in the case of some demons) their sins.

The devil still uses his formal titles of "Light bringer" and "Accuser" because although he is currently opposed to heaven, he still fights against heaven's enemies and persecutes sinners. Very complicated situation, but as far as he's concerned, God betrayed HIM.

Michiael means "He who is like God" so it should be oubvious why the Devil wouldn't use it anymore. You could even argue that giving him that title in the first place was what set him up for failure. How can one be LIKE God while still be subservient to him?

Light bringer, Accuser, he was god's greatest warrior, and fought to defend heaven from "the impurity" that existed outside god's perfect creation. But as his campaigns took him farther and farther into what is now hell, he was granted more and more power and authority.

Eventually he got strong enough to chalange God's claim to be the one true God. After all, God was now dependent on his angels, and the angels now answered to Michael. (Who is like god)

Eventually god started to feel threatened, and decided to accuse the accuser of treason.

Any other angel would have gladly died if God told him to, but at this point the light bringer had obtained a sense of self preservation. He found himself unable to obey god and instead compelled to defend himself. This was extremely difficult because he loved god so much.

Ultimately however, his love for himself proved to be stronger than his love for a God that had created him to suffer in such a way. He became heaven's worst possible enemy, and retroactively justified God's fear of him. If god could not tolerate him, he would take his place.

Convincing other angels to join his rebellion was easy. After all, he was the light bringer, and could give anybody the gift of knowledge of good and evil. Humanity was collateral damage, god never intended us to have this ability, hence the necessity of Christ's sacrifice.

There is no original sin, god never intended to allow us into heaven, and we were granted free will by "the enemy" as part of an extended proxy war against heaven. Christ represents a reform movement to redeem humanity for the sin of existing outside of God. The only real sin.

Jesus and satan are sympathetic to each other's goals, but think the other is misguided and self defeating.

Jesus' plan to admit humanity into heaven would effectively make them no better than angels. Satan instead plans to replace God fully aware it would make him a hypocrite.

Save humanity vs rule as a benevolent dictator over a universe where humanity does not need salvation.