As a Christian, I cannot imagine any answer to the question of evil likely to satisfy an unbeliever; I can note, though, that--for all its urgency--Voltaire's version of the question is not in any proper sense "theological." The God of Voltaire's poem is a particular kind of "deist" God, who has shaped and ordered the world just as it now is, in accord with his exact intentions, and who presides over all its eventualities austerely attentive to a precise equilibrium between felicity and morality. Not that reckless Christians have not occasionally spoken in such terms; but this is not the Christian God.
The Christian understanding of evil has always been more radical and fantastic than that of any theodicist; for it denies from the outset that suffering, death and evil have any ultimate meaning at all. Perhaps no doctrine is more insufferably fabulous to non-Christians than the claim that we exist in the long melancholy aftermath of a primordial catastrophe, that this is a broken and wounded world, that cosmic time is the shadow of true time, and that the universe languishes in bondage to "powers" and "principalities"--spiritual and terrestrial--alien to God. In the Gospel of John, especially, the incarnate God enters a world at once his own and yet hostile to him--"He was in the world, and the world was made by him, and the world knew him not"--and his appearance within "this cosmos" is both an act of judgment and a rescue of the beauties of creation from the torments of fallen nature.
Whatever one makes of this story, it is no bland cosmic optimism. Yes, at the heart of the gospel is an ineradicable triumphalism, a conviction that the victory over evil and death has been won; but it is also a victory yet to come. As Paul says, all creation groans in anguished anticipation of the day when God's glory will transfigure all things. For now, we live amid a strife of darkness and light.
When confronted by the sheer savage immensity of worldly suffering--when we see the entire littoral rim of the Indian Ocean strewn with tens of thousands of corpses, a third of them children's--no Christian is licensed to utter odious banalities about God's inscrutable counsels or blasphemous suggestions that all this mysteriously serves God's good ends. We are permitted only to hate death and waste and the imbecile forces of chance that shatter living souls, to believe that creation is in agony in its bonds, to see this world as divided between two kingdoms--knowing all the while that it is only charity that can sustain us against "fate," and that must do so until the end of days.
I look at all of this in several ways. Number one, this world is not the Happy Ending, and we shouldn't be shocked when it doesn't act like it. The Happy Ending (Heaven) is something we should thirst for and strive to reach.
Number two, the bell tolls for all of us. Suffering isn't additive in such a way that a huge body count in a single event makes the world a more evil place. If suffering worked that way, wouldn't we be able to say "better that one extra person die today, than that 100 million painfully stub their toes", assuming the pain of death is less than 100 million times as intense as the pain of stubbing a toe? One person can only experience one person's worth of suffering (which includes empathy and grief for the suffering and loss of others). No one, except for God in his omniscience, has any direct experience of "collosal cumulative suffering". No single merely human person feels the weight of all suffering in the universe. We should be just as incensed at the fact that a single particular person can lose his youthful vitality, growing old, senescent, feeble, and infirm, and eventually dying in great pain, as we are about these huge disasters. Death on any scale is an outrage that cries out to God for redemption.
Third, Eternity gets the final Word. All's well that ends well. If God ultimately achieves the glorious victory of a new Heaven and a new Earth, where "every tear is wiped away", and allowing St. Paul to say "I consider the present suffering to be as nothing compared to the glory that awaits us", then in the grand scheme, there is no injustice being done.
Finally, if it does all seem like a horrific injustice, where God must be some sort of evil, tyrannical, uncaring SOB for letting it all happen, maybe he deserves some sort of severe punishment. Maybe we need to tie Him to a post, flog Him, spit on Him, ridicule Him, shove a crown of thorns on His head, make Him stagger under the weight of a cross, nail Him to it, and jeer at Him as He dies. Maybe that would even the score...