When you grow up reading a lot of genre fiction, especially young adult and high fantasy, a major turning point in your emotional growth is realizing that “the dark lord”, as you have come to know this all-too-common character archetype, doesn’t really exist. In reality, evil as an ideal is never made manifest in a single adversary whose sole objective is to destroy and corrupt the goodness in the world. Sure, there are people who are “bad” from your own perspective, and bad qualities like selfishness, prejudice, and lack of empathy are generally culturally agreed upon, but even the worst people are generally heroes in their own minds, people who have not yet been shown the error of their ways. No one sets out to be Sauron or the White Witch or Voldemort, and no matter how much power and influence bad people achieve, I know of no instance where anyone has claimed that their ultimate goal was the advancement of the cause of evil.
Most frameworks of morality grasp this concept pretty well: that good and evil are not absolutes, and that humans inherently have the capacity for both positive and negative behaviors. The major exception seems to be in certain camps of modern Christianity, which assigns a motive and influence to Satan that is very much comparable to the fictional and largely metaphorical presence of Sauron and other prototypical “dark lords”. While in Tolkien’s case, Sauron was a metaphor for industrialization, and in the case of children’s books, morality is artificially externalized and simplified for the sake of young readers, the Christian reading of Satan is—as far as many active faith communities are concerned—neither metaphorical nor exaggerated. Satan is literally a dark wizard.