Dorothy Sayers expresses well and in clear vernacular the reason for Christmas, the purpose of the incarnation: "God chose to make man as he is—limited and suffering and subject to sorrows and death—He [God] had the honesty and the courage to take His own medicine."
There are many rational and sound ways to answer questions about why God allowed evil. In the end, God answers with Himself. God subjected Himself to the evil in this world and suffered the consequences. He isn't a remote God observing human frailty; He's experienced it, too. He "took His own medicine" by living a real human life and then suffering a hideous death He didn't deserve. When we cry out about evil, He knows exactly what we're feeling.
The little baby was born in a manger to experience evil and suffer in order to give us the final answer to it.
The Church’s answer is categorical and uncompromising and it is this: That Jesus Bar-Joseph, the carpenter of Nazareth, was in fact and in truth, and in the most exact and literal sense of the words, the God “by whom all things were made.” His body and brain were those of a common man; his personality was the personality of God, so far as that personality could be expressed in human terms. He was not a kind of demon pretending to be human; he was in every respect a genuine living man. He was not merely a man so good as to be “like God”; he was God.
Now, this is not just a pious commonplace; it is not commonplace at all. For what it means is this, among other things: that, for whatever reason, God chose to make man as he is—limited and suffering and subject to sorrows and death—he [God] had the honesty and the courage to take his own medicine. Whatever game he is playing with his creation, he has kept his own rules and played fair. He can exact nothing from man that he has not exacted from himself. He has himself gone through the whole of human experience, from the trivial irritations of family life and the cramping restrictions of hard work and lack of money to the worst horrors of pain and humiliation, defeat, despair, and death. When he was a man, he played the man. He was born in poverty and died in disgrace, and thought it was worthwhile.
Christianity is, of course, not the only religion that has found the best explanation of human life in the idea of an incarnate and suffering god. The Egyptian Osiris died and rose again; Aeschylus in his play, The Eumenides, reconciled man to God by the theory of a suffering Zeus. But in most theologies, the god is supposed to have suffered and died in some remote and mythical period of prehistory. The Christian story, on the other hand, starts off briskly in St. Matthew’s account with a place and a date: “When Jesus was born in Bethlehem of Judea in the days of Herod the King.” St. Luke, still more practically and prosaically, pins the thing down by a reference to a piece of government finance. God, he says, was made man in the year when Caesar Augustus was taking a census in connection with a scheme of taxation. Similarly, we might date an event by saying that it took place in the year that Great Britain went off the gold standard. About thirty-three years later (we are informed), God was executed, for being a political nuisance, “under Pontius Pilate”—much as we might say, “when Mr. Johnson-Hicks was Home Secretary.” It is as definite and concrete as all that.
This is an excerpt from "The Greatest Drama Ever Staged," which you can read here.