From The Problem of Pain, Chapter 7:
"The fact that God can make complex good out of simply evil does not excuse-though by mercy it may save- those who do the simply evil. And this distinction is central. Offences must come, but woe to those by whom they come; sings do cause grace to abound, but we must not make that an excuse for continuing to sin. The crucifixion itself is the best, as well as the worst, of all historical events, but the role of Judas remains simply evil. We may apply this first to the problem of other people's suffering. A merciful man aims at his neighbor's good and so does 'God's will; consciously co-operating with 'the simple good.' A cruel man oppresses his neighbor, and so does simple evil. But in doing such evil, he is used by God, without his own knowledge of consent, to produce the complex good-so that the first man serves God as a son, and the second as a tool. For you will certainly carry our God's purpose, however you act, but it makes a difference to you whether you serve like Judas or like John."
I highly recommend the book, and one of these days I will outline and enumerate on it here. Perhaps a nice long post on the 'Hell' chapter might be in order!