"God certainly is strong enough to obliterate evil from the earth or to have prevented it in the first place. No question about that. But let me ask you a question. Is it a good thing that God created human beings as free moral creatures, capable of making moral choices? It strikes me that the answer to that is yes. Because God is good--which is one of the things in question here--God created free moral creatures.
But this changes everything, doesn't it? What makes you think that strength has anything to do with God creating a world in which there are genuinely free moral creatures and no possibility of doing wrong?
You see, now we're back to square circles. It's just as ridiculous to ask God to create a world in which we have genuinely free creatures with no possibility to do wrong, as it is to ask Him to create a square circle. The task has nothing to do with His strength. It has to do with the nature of the problem. If you're going to have morally free creatures--that is, human beings that can make moral choices for themselves--and if God is good, then He is going to create creatures that will be truly morally free. But that entails, of necessity, at least the possibility of evil in the world."
Greg Koukl - The strength of God and the problem of evil