One of the reasons non-believers give for their unbelief in God is the presence and proliferation of evil in the world. They view evil as a blight against the claims of the Christians about the goodness of their God. Very often the question advanced is: “How can there be a good God when there is so much evil in the world?” This question is veiled with a multitude of assumptions and presumptions within itself. But it becomes mind-boggling when it is asked by people who believe that human beings are a product of a mindless, unguided natural process living in a universe that has “no design, no purpose, no evil, and no good…” and that they merely “dance to the music” of their DNA. [1] In such a closed system of mindlessness with an existence that is devoid of cognitive functions, who can qualify to recognize evil, let alone to ask questions about it?
But even theists and Christians struggle with the question of evil. It is an enigma that is hard to deal with because we do not have all the answers about it yet. But we can know something about it by studying and drawing inferences from what Scriptures say.
What indeed is evil? Evil is good gone bad. It is the corruption of that which was originally good. Evil cannot exist alone. Thomas Aquinas argues that God created everything, but evil is not a tangible thing that can exist on its own as a stand-alone entity. This is not say that evil is unreal. Rather, it is to say that evil has the potential to exist in a parasitic nature in some substance that is good: “evil signifies nothing else than ‘privation of perfect being.’” [2]. When we speak of moral evil, we are talking about a relationship between human beings that has been corrupted.
God created perfect human beings. But he created them with a freewill. They were created with the capacity to love and obey God or to reject and disobey him. God did not create robots who mechanically obey him. C.S. Lewis aptly observed that God took the risk of endowing the creatures he created with a freewill because coerced love and obedience are meaningless to him. God put his image in the creatures in whom he also deposited the power to choose either to do good or to do evil. No single human being can appreciate a robot manufactured and programed to say “I love you” every 2 hours! If someone did that, they would merely be telling themselves they love themselves which smacks of narcissism. Nothing is so gratifying and heartwarming than to be the object of free, warm and uncoerced love.
But why can’t God just stop evil in the world? The only way God can stop evil in this world is to take away from mankind the freewill he gave us; to take away the capacity to choose to love him and all he stands for or to choose to reject him and all he stands for. He would have to go back on his word of creating people with freewill and make them into some form of puppets.
For those who argue that God could have done a better job at creating this world, here is what Lewis says (and I agree with him, totally): “Of course God knew what would happen if [human beings] used their freedom the wrong way: apparently He thought it worth the risk. Perhaps we feel inclined to disagree with Him. But there is a difficulty in disagreeing with God. He is the source from which all reasoning power comes: you could not be right and He wrong any more than a stream can rise higher than its own source. When you are arguing against Him you are arguing against the very power that makes you able to argue at all: it is like cutting off the branch you are sitting on. If God thinks this state of war in the universe a price worth paying for free will-that is, for making a live world in which creatures can do real good or harm and something of real importance can happen, instead of a toy world which only moves when He pulls the strings-then we may take it that it is worth paying.”[3]

Resources
[1] Richard Dawkins, Out of Eden, pp. 133.
[2] Compendium theologiae 114, 125-126; In Bill Kin (2002). Thomas Aquinas on the Metaphysical Problem of Evil. Quodlibet Journal, 4, (2-3). ISSN: 1526-6575
[3] Lewis, Mere Christianity, pp. 52-53