Robert Adams raises and interesting objection to modal realism based on the problem of evil. He believes
[That] our very strong disapproval of the deliberate actualizing of evils… reflects a belief in the absolutely, and not just relatively, special status of the actual as such. Indeed, if we ask, “What is wrong with actualizing evils, since they will occur in some other possible world anyways if they don’t occur in this one?”, I doubt that the indexical theory can provide an answer which will be completely satisfying ethically.
Adams’ objection concerning the actualization of evil is irrelevant to a Thomistic version of modal realism (this version to be released in an upcoming paper in the Fall of 2012). Thomas does not seem to have any problem with the presence of evil. When discussing Boethius, a philosopher prompts the question, “If there is a God, how comes evil?” Thomas argues that the question should be reversed—“If there is evil, there is a God.” For there would be no evil, if the order of goodness were taken away, the privation of which is evil; and this order would not be, if God were not. Following Augustine’s lead on evil and privation he then argues for a soul-making notion of evil. There are many good things in creation, which would find no place there, unless evils were there also. Thus, many virtues would be absent. In the physical order, one thing cannot come to be unless something else is destroyed. If then evil were wholly excluded from the universe by providential means, the number of good things would be proportionally diminished: which ought not to be, because good is more vigorous in goodness than evil in badness.
It is certainly the case that even in a plurality of worlds Thomas would have no objection on the basis of evil no matter how much evil is present. However, suppose that the privation and soul-making defense do not suffice. No matter what the reason is, if God has a sufficient reason for permitting evil in some possible world then he has a sufficient reason in all possible worlds. Given simplicity, God is perfectly similar in every possible world we can conceive. He never wills differently, he never acts differently, he never knows differently, and he never loves differently. If modal realism is true and evil exists then the probability overall or on balance for justice is precisely 1. Thus, the problem of evil is an insufficient objection given whatever God’s interaction is in this world it would be morally equivalent to his actions in other worlds with evil. If God is absolutely similar in all possible worlds and if he has a morally sufficient reason to permit evil in some possible world then he is morally justified in permitting evil in all possible worlds (even if some worlds are more bad than good because God would be acting towards the same telos). The following is a modified version of Alvin Plantinga’s ontological argument. In it I include the necessary entailment of a morally sufficient reason for permitting evil.
P1. The property of being maximally great is exemplified in some possible world.
P2. The property of being maximally great is equivalent, by definition, to the property of being maximally excellent in every possible world.
P3. The property of being maximally excellent entails the properties of omniscience and moral perfection.
P4. The property of moral perfection necessarily entails a morally sufficient reason for permitting states of affairs that are overall more evil than good.
P5. A universal property is one that is exemplified in every possible world or none.
P6. Any property that is equivalent to some property that holds in every possible world is a universal property.
C1. There exists a being that is essentially omniscient and morally perfect.
C2. There exists a being that has essentially morally sufficient reasons for permitting states of affairs that are overall more evil than good.
 Robert Adams, “Theories of Actuality” in The Possible and the Actual ed. Michale Loux (Ithaca, NY: Cornell University Press, 1979), 195.
 Thomas, Summa Contra Gentiles 3.71.
 William Lane Craig, Time and Eternity (Wheaton, IL: Crossway, 2001), 89.
 This will function in the same way the ontological argument works. If it is possible for God to exist in some world then he exists in all worlds. Similarly, if God is simple and absolutely similar in all worlds then if he has a morally sufficient reason for allowing evil in some possible world then he has a morally sufficient reason for allowing it in all possible worlds.
 Robert E. Maydole, “The Ontological Argument” in The Blackwell Companion to Natural Theology eds. William Lane Craig and J.P. Moreland (Oxford: Blackwell, 2009), 573.