"A world that had never been touched by evil would be a good place, but it wouldn't be the best place possible. The best of all worlds would be a place where evil facilitated the development of virtues that are only able to exist where evil flourishes for a time. This would produce a world populated by souls that were refined by overcoming evil with good. The evil is momentary. The good that results is eternal.
What good comes out of a drive-by killing, someone might ask, or the death of a teenager through overdose, or a daughter's rape, or child abuse? The answer is that a commensurate good doesn't always come out of those individual situations, though God is certainly capable of redeeming any tragedy. Rather, the greater good results from having a world in which there is moral freedom, and moral freedom makes moral tragedies like these possible."
-Greg Koukl, Augustine on evil