C.S. Lewis, Natural Law, Witches, & the Death Penalty

I've just started reading C.S. Lewis' Mere Christianity. So far, it's a series of short chapters about Natural Law, which Lewis refers to in various forms (Rule of Decent Behaviour, Law of Behaviour, Moral Law, &c.). At the end of one chapter there is a collateral discussion about the proper use of the death penalty:

"I have met people who exaggerate the differences, because they have not distinguished between differences of morality and differences of belief about facts. For example, one person said to me, "Three hundred years ago people in England were putting witches to death. Was that what you call the Rule of Human Nature or Right Conduct?" But surely the reason we do not execute witches is that we do not believe there are such things. If we did – if we really thought that there were people going about who had sold themselves to the devil and received supernatural powers from him in return and were using those powers to kill their neighbors or drive them mad or bring bad weather – surely we would agree that if anyone deserved the death penalty, then these filthy quislings did? There is no difference in moral principle here: the difference is simply about matter of fact. It may be a great advance in knowledge not to believe in witches: there is no moral advance in not executing them when you do not think they are there. You would not call a man humane for ceasing to set mousetraps if he did so because he believed there were no mice in the house."