Popular Christianity is heavily based on paganism, which is incredibly ironic considering that paganism is generally posed as the antithesis of Christianity. The story of Lucifer is syncretized with the story of Prometheus, although Lucifer doesn't really benefit humanity at all. According to the popular interpretation, Lucifer is the origin of all evil, became a snake in the garden of Eden, and then tempted Eve to eat the fruit of the tree of the knowledge of good and evil. However, the snake isn't actually connected to Lucifer in the text—that interpretation was added later to explain the problem of evil (why it exists if God is supposedly good)

The idea that Lucifer is insubordinate and violated the natural hierarchy is very old, but the idea that Lucifer is the origin of evil is relatively new.

Christian theology contains many holes like this because there's a tendency towards treating every word in the Bible as literal, where it may have been written allegorically or as a parable, as Jesus often did. (Just to be clear, Jesus did NOT write the Bible, I'm just pointing out that the writers of the Bible may have tried to replicate his style.) This issue is compounded when you include the Old testament, as it contains portions which are clearly mythological, but are nonetheless treated as fact by certain modern Christians.