Is God willing to prevent evil, but not able?
Then he is not omnipotent.
Is he able, but not willing?
Then he is malevolent.
Is he both able and willing?
Then whence cometh evil?
Is he neither able nor willing?
Then why call him God? — Epicurus
I consider this one of the best atheist quotes of all time. It says so much, so well, in so few words. 2,300 years of failed theodicy apologetics later, no theist has yet managed to come up with a satisfactory reply.