To review human reflection about evil is to review the entire history of theology, philosophy, religion, and literature, from the Rig Veda to Plato to Dostoyevski to Wittgenstein. And to consider the effective operations of evil in human life is to consider the whole history of mankind, from paleolithic tribes to us now.
There are two ways--each of them with a number of variants--that philosophers, theologians, scholars, scientists, and ordinary people have tried, throughout the centuries, to cope with the so-called problem of evil. As with all important human issues, we can try either to solve the "problem" or to get rid of it altogether by declaring it invalid: by denying that the problem exists. Among those who tried to tackle the problem, we find adherents of two fundamentally opposite (or so it seems) metaphysics: Manichaeans and Christians. Among those who denied the validity of the problem--though not all of them for the same reason--there are some mystics, some pantheists, all Marxists and Communists, most other utopians, and most advocates of a naturalistic world-view, like Nietzscheans, Nazis, and philosophical Darwinists.
It is trivially true that the concept of evil as pure negativity is a simple deduction from the belief in a creator who is both unique and infinitely good, so that whatever is, is good necessarily, and existence as such is good. This, to repeat, is a logical deduction, not a matter of experience. But it is mostly through such arguments that Christian theodicy has made its enormous, indeed heroic, efforts to respond to the most common experience of ordinary people--the experience of evil. When St. Augustine says that the very presence of evil must be good because if it were not, God would not have allowed evil to appear, he is stating something that is obvious in Christian terms. This, again, is a logical deduction from the concept of God; it implies that God could, if he wanted, prevent evil from appearing, but for reasons best known to Himself preferred to let it stay.
Leibniz is more specific in explaining what those reasons may have been. He, too, does so by deduction. Having proved the necessary, existence of God and, separately, His supreme goodness, Leibniz inferred from these that God must have created the best world that is logically conceivable, and that this is the world we inhabit; any other world would be worse.
Voltaire's famous derision of this idea is too easy. Leibniz was well aware of the horrors of life. Nevertheless, belief in the supreme goodness of creation is irresistible given such an idea of the divine being. And it implies that God, in His all-embracing wisdom, must have solved, as it were, an equation in an infinitely complex higher calculus, in order to calculate which world would produce the maximum goodness. Christian tradition has always stressed, after Plato, the distinction between moral evil and suffering: moral evil--malum culpae--is the inevitable result of human (and angelic) free will, and Leibniz's creator reckoned that a universe populated by rational creatures endowed with free will, and thus capable of doing evil, would generate more good than a world whose dwellers would, in effect, be automata, programed never to do evil (and presumably, though Leibniz does not say, so explicitly, never to do good either, if by a good action we mean, as we commonly do, an action done out of choice, not from compulsion).
As for non-man-made suffering--malum poenae in the Christian idiom--there are two possible answers. One says that it is the work of malevolent spirits whose work is permitted by God in order to punish us, correct us, warn us, etc. The other, in the Leibnizian spirit, says that such suffering results from the workings of the laws of nature and that God is not omnipotent in the sense that He can combine everything with anything and impose on the physical universe an order where things would not move according to strict regularities--would stop interfering and colliding with each other. …