There’s a good version of the modal problem of evil in Ted Guleserian’s (TG), ‘God and Possible Worlds: The Modal Problem of Evil’ (GPW) in Nous (1983). GPW is directly largely to Plantinga’s modal realism+theism and similar views. But I think the problem is more difficult than he suggests. TG tries to show that there is a possible world in which there is pointless and preventable evil. And so he invites a response of modal skepticism about such a world. He would have been better advised to provide a series of worlds, a G series and a B series, and then ask how the evil in the B series could be necessary to a greater good: i.e., how the evil in the B series could be justified evil.
It’s a common view among theists that there is no best possible world. This position helps to explain, among other things, why our world is not the best possible. Instead, there is a series of worlds, w0, w1, w2, . . ., wn of better and better worlds with no upper bound on positive value. w0 might include n happy people (or n completely fulfilled people or n people experiencing the beatific vision, etc). w1 includes n + 1 happy people, w2 include n + 2. And so on upward. There’s an addition argument here that’s hard to resist; worlds can get better in this respect, no matter how good. Call that the G series.
There is also a series, w-1, w-2, w-3, . . ., w-n of worse and worse worlds with no lower bound on disvalue. w-1 includes n suffering people, w-2 includes n + 1 suffering people, and so on downward. There’s an addition argument here that’s hard to resist; worlds can get worse in this respect, no matter how bad they are. Call that the B series.
The modal argument of evil charges that the evil in, say, w-3, is not necessary to a greater good, since, obviously, there is a better world in the B series–w-2, say–that includes less suffering and is better. Of course, this understates the problem. There is a much better world in the G series that includes much less suffering and evil. So, clearly, the evil in the B series is not necessary to any greater good.
This is a much more difficult version of the modal argument from evil. It is a version that retreating to a credible form of modal skepticism won’t help. This is not to say, of course, that I don’t think there is a solution.