Part of Lewis's chapter on Hell from the Problem of Pain

Without hell, can evil triumph?

First, there is an objection, in many minds, to the idea of retributive punishment as such. This has been partly dealt with in a previous chapter. It was there maintained that all punishment became unjust if the ideas of ill-desert and retribution were removed from it; and a core of righteousness was discovered within the vindictive passion itself, in the demand that the evil man must not be left perfectly satisfied with his own evil, that it must be made to appear to him what it rightly appears to others--evil. I said that Pain plants the flag of truth within a rebel fortress. We were then discussing pain which might still lead to repentance. How if it does not--if no further conquest than the planting of the flag ever takes place? Let us try to be honest with ourselves. Picture to yourself a man who has risen to wealth or power by a continued course of treachery and cruelty, by exploiting for purely selfish ends the noble motions of his victims, laughing the while at their simplicity; who, having thus attained success, uses it for the gratification of lust and hatred and finally parts with the last rag of honour among thieves by betraying his own accomplices and jeering at their last moments of bewildered disillusionment. Suppose further, that he does all this, not (as we like to imagine) tormented by remorse or even misgiving, but eating like a schoolboy and sleeping like a healthy infant--a jolly, ruddy-checked man, without a care in the world, unshakably confident to the very end that he alone has found the answer to the riddle of life, that God and man are fools whom he has got the better of, that his way of life is utterly successful, satisfactory, unassailable. We must be careful at this point. The least indulgence of the passion for revenge is very deadly sin. Christian charity counsels us to make every effort for the conversion of such a man: to prefer his conversion, at the peril of our own lives, perhaps of our own souls, to his punishment; to prefer it infinitely. But that is not the question. Supposing be will not be converted, what destiny in the eternal world can you regard as proper for him? Can you really desire that such a man, remaining what he is (and he must be able to do that if he has free will) should be confirmed forever in his present happiness should continue, for all eternity, to be perfectly convinced that the laugh is on his side? And if you cannot regard this as tolerable, is it only your wickedness--only spite--that prevents you from doing so? Or do you find that conflict between justice and Mercy, which has sometimes seemed to you such an outmoded piece of theology, now actually at work in your own mind, and feeling very much as if it came to you from above, not from below? You are moved, not by a desire for the wretched creature's pain as such, but by a truly ethical demand that, soon or late, the right should be asserted, the flag planted in this horribly rebellious soul, even if no fuller and better conquest is to follow. In a sense, it is better for the creature itself, even if it never becomes good, that it should know itself a failure, a mistake. Even mercy can hardly wish to such a man his eternal, contented continuance in such ghastly illusion. Thomas Aquinas said of suffering, as Aristotle had said of shame, that it was a thing not good in itself, but a thing which might have a certain goodness in particular circumstances. That is to say, if evil is present, pain at recognition of the evil, being a kind of knowledge, is relatively good; for the alternative is that the soul should be ignorant of the evil, or ignorant that the evil is contrary to its nature, "either of which," says the philosopher, "is manifestly bad." (Summa Theol., 1, 11ae,Q. xxxix, Art. 1.) And I think, though we tremble, we agree.