As one would expect with C.S. Lewis, author of The Chronicles of Narnia, one of the main themes of The Magician's Nephew is the never-ending struggle between good and evil. As a devout Christian, Lewis believed that this epic conflict was real and ongoing and that all men and women of good will should constantly fortify themselves against the ever-present threat of Satan's devious wiles.
Unfortunately, Digory's Uncle Andrew fails to grasp this simple warning and foolishly messes around with the forces of darkness through his experiments in magic. Uncle Andrew is by no means an evil man, but he unwittingly unleashes evil forces on an unsuspecting world by playing around with something he doesn't fully understand. In that sense, he's an enabler of evil, even if not exactly wicked himself.
For an embodiment of sheer, unadulterated wickedness, however, we must turn to Queen Jadis, the White Witch herself. She is the antagonist in the story and represents an ever-present threat to the children, as indeed she does to the lion Aslan, who is an allegory of Christ. Though Jadis is defeated by Digory's refusal to give in to temptation she has not been completely vanquished. This is Lewis's way of saying that the struggle between good and evil goes on, and that the faithful must always be on their guard against the dangerous lure of the devil and all his temptations. That Jadis still remains a threat also foreshadows her reappearance in The Lion, The Witch, and The Wardrobe.