This is one of the big ones, of course, and if there was a straightforward or easy answer someone -- Irenaeus, Aquinas, whoever -- would have come up with it. The problem is contained in the assumptions in the question: 'a good God' and the like. We are never, repeat never, in a position where we can size up God and decide what such a being ought really to do. A lot of people today assume, vaguely, that God ought to be running things, stopping earthquakes, preventing road accidents, whatever. They seldom stop to imagine what their own world might be like if God really stepped in every time we were about to do something wrong.

The Bible doesn't pose, or answer, the question that way. It tells a long, complex narrative about a plan launched by the creator God to heal creation. This plan, begun with the call of Abraham, reaches its climax in Jesus and his horrific death, and works out from there, not to the rescue of souls from a doomed world, but to the healing and renewal of the whole creation. That is the framework within which we may be able not indeed to answer the question as posed (which is actually a very post-Enlightenment way of putting it: see Susan Neiman's brilliant book, "Evil in Modern Thought"), but to grapple with the actual world in which evil remains so powerful yet Jesus and his followers declare that the creator God is becoming king.

On all this (sorry for the plug) see my book "Evil and the Justice of God."