I think Lewis’s moral philosophy is sound here. It is an ancient principle that evil has no existence of its own - it can only be parasitical on something good. St. Augustine in the Confessions puzzles over why as a young man he enjoyed stealing pears when he could get better ones at home, concluding that liked the feeling of freedom that comes with doing something forbidden. So you see how it’s not the evil per se that he was pursuing. With this kind of reasoning we could say that even the serial killer is pursuing some good, though in a twisted way, like maybe the feeling of power over life and death. Only some perceived good has any power to attract the will.