This short essay by Granville Sewell is I think one of the best deistic rationalizations of the reality of evil I have encountered. Of course there are other rationalizations, and of course the materialist view that no rationalization is possible, so "suck it up".
A vast amount of suffering is caused by evil actions of human beings. Second, there is a vast amount of "natural evil" caused by the natural world by things like disease, floods and earthquakes. Any proposed deistic or other solution to the ancient theological problem of suffering has to explain both categories.
The basic approach in this essay is to combine various arguments that mankind's suffering is an inevitable accompaniment of our greatest blessings and benefits.
Why pain, suffering and evil? Main points that are made:
(1) There is the observed regularity of natural law. The basic laws of physics appear to be cleverly designed to create conditions suitable for human life and development. It can be surmised that this intricate fine-tuned design is inherently a series of tradeoffs and balances, allowing and fostering human existence but also inevitably allowing "natural evil" to regularly occur. In other words, the best solution to the overall "system requirements" (which include furnishing manifold opportunities for humans to experience and achieve) inherently includes natural effects that cause suffering to human beings.
This points out that there may be logical and fundamental limitations to God's creativity. Maybe even He can't 100% satisfy all the requirements simultaneously. Maybe He doesn't have complete control over nature, because that would interfere with the essential requirements for creative and fulfilling human life. After all, human achievement requires imperfection and adverse conditions to exist as a natural part of human life.
(2) There is the apparent need for human free will as one of the most important "design requirements". This inevitably leads to vast amounts of suffering caused by evil acts of humans to each other. Unfortunately, there is no way to get around that one, except to make humans "zombies" or robots.
(3) Some suffering is necessary to enable us to experience life in its fullest and to achieve the most. Often it is through suffering that we experience the deepest love of family and friends. "The man who has never experienced any setbacks or disappointments invariably is a shallow person, while one who has suffered is usually better able to empathize with others. Some of the closest and most beautiful relationships occur between people who have suffered similar sorrows."
Some of the great works of literature, art and music were the products of suffering. "One whose life has led him to expect continued comfort and ease is not likely to make the sacrifices necessary to produce anything of great and lasting value."
Of course, the brute fact is that there is an egregious amount of truly innocent and apparently meaningless suffering, that our instinct tells us is wrong. Is it at all worth it?
Sewell concludes:
"Why does God remain backstage, hidden from view, working behind the scenes while we act out our parts in the human drama? ....now perhaps we finally have an answer. If he were to walk out onto the stage, and take on a more direct and visible role, I suppose he could clean up our act, and rid the world of pain and evil — and doubt. But our human drama would be turned into a divine puppet show, and it would cost us some of our greatest blessings: the regularity of natural law which makes our achievements meaningful; the free will which makes us more interesting than robots; the love which we can receive from and give to others; and even the opportunity to grow and develop through suffering. I must confess that I still often wonder if the blessings are worth the terrible price, but God has chosen to create a world where both good and evil can flourish, rather than one where neither can exist. He has chosen to create a world of greatness and infamy, of love and hatred, and of joy and pain, rather than one of mindless robots or unfeeling puppets."
Overall, it all may be a vast tradeoff, and some people might conclude it isn't a good one from the human perspective.