The problem of evil is the most popular objection to God's existence. And not just among village atheists, but philosophers. If an omniscient, omnipotent, and benevolent God truly existed, he'd either prevent evil or step in before things become horrific.
There is, though, something to be said for God letting evil run its course some of the time. There's a scene from a documentary about Nazi propagandist Riefenstahl (The Wonderful Horrible Life of Leni Riefenstahl) in which she and her surviving camera crew return to the site of the 1936 Olympiad, which she immortalized in one of her infamous films. This was shot during the heyday of the Third Reich, when the Nazis seemed to be unstoppable. But, of course, the viewer perceives the scene in hindsight, as does Riefenstahl. This was the apogee of human hubris, but the stark contrast between the Titanic megalomania of Germany in 1938, and its humiliating, devastating defeat, would be a lesson lost if God had intervened. Or, to take a more recent example, consider the situation of Venezuela after Chavez.
Many people ignore or mock dire warnings as alarmism. They don't take evil seriously. It's only when things it bottom, when they see it for themselves, that the unwelcome truth finally sinks in.
Of course, that's a very hard lesson for all concerned parties, but that's mitigated by eschatological justice and eschatological compensations.