I haven't actually, but thank you for mentioning it. I didn't know Lewis had a Space series. The theological reasoning has to do with the fact that God created all things "good" and absolute evil, having not been created, does not exist, but evil would be defined as a corruption of something good. Patristic fathers deal with the problem of sin and evil quite differently than the western understanding of original sin and guilt imputation. They would say that mankind is fallen in their nature, guilty of sin by their own faults and weaknesses compromised, and yet there is hope in healing those areas and bringing them into subjection to the Lordship of Christ. Thus we are not fighting a hopeless fight against sin or with individuals who seem too far gone, but praying for Christ's mercy to shed upon the brokenness to make us all more and more like Himself. Quite a beautiful picture actually.