Can somebody please explain why Satanism is listed under paganism, when Satan is one of the Christian deities?

Because way back when the church conflated Satan and pagan beliefs and to this day the average person doesn't understand they have nothing to do with each other.

Because... The Miltonian myth of Satan as an angel named Lucifer who rebelled against God and was cast out of Heaven in primordial times has no real grounding in the Bible, and yet it is the origin story that many—if not most—Christians regard as canonical. In the Old Testament, Satan operates as an obedient member of God’s heavenly court even as he roams the earth testing God’s followers. In Jubilees, he is the leader of the evil spirits who remain after the flood, permitted by God to tempt humans. Early Christianity incorporates Satan into a Middle Platonist matrix, imagining him to be the prince of the corrupt angels or demons who control the earth and lowest heavens. It is only the tradition we find in an apocryphal text, The Life of Adam and Eve, that moves Satan’s expulsion back to Eden and explains why it happened.

Many academics in the field agree that Satan and Lucifer are a confabulation of different myths, as was common in ancient literature and occurs even today. Nowhere in the Bible does it say that Lucifer is Satan. And Satan has never been considered a deity in Christianity. It's not the Marvel Universe with villain Gods and dark deities.

https://isthatinthebible.wordpress.com/2021/10/28/when-and-why-does-satan-fall-in-luke-1018/

Satanism isn't about worshipping Satan, but to worship ones self as in do whatever makes you happy.

Christian’s tend to mark any religion that isn’t Abrahamic as “pAgAn”.

Satanism isn't even Satan-centric, it's more about living for this world instead of the pipedream provided by religion. It's liberating when compared to most forms of Christianity.

Most Christians don't think he is a christian deity either. But he is in their pantheon, they are Christians and believe in him so... 🤷🏻