How to explain Satan? What could be harder to believe than an angel deciding to turn from God’s goodness and do evil until the end of time? Yet what could be harder to deny than the existence of persistent evil in this world?
I know that angels could pursue evil because being has nothingness as its counterpart and they could freely choose this nothingness, which is evil (evil, strictly speaking, does not exist – it is the absence of goodness). But why was Satan so insistent in his evil? I know that it is part of the infinite goodness of God that He allows evil to exist and out of it extracts good (Summa Theologiae, 1.2.3). Very well, we know why God allows Satan to be evil, but why does he have to be evil eternally, and cast into hell for eternity? Would it not be the greatest good of all for even the devil to repent and be restored to God? We know that God cannot force one into blessedness, for one must be contrite to be perfectly blessed (i.e., attain heaven), and for all we know Satan will never be contrite. Perhaps he is like some men who chose evil precisely because it is evil, and they get a sick pleasure out of doing so, and sometimes they never tire of it. Although God disposes all of us towards the good, he allowed us to choose to take evil as our good.
But surely even the most wicked demon will come to his senses and realize that the greatest goodness is not found in evil? Surely then the demon would become contrite and be able to achieve perfect bliss? Perhaps, but even then God may not allow him to attain heaven. True, some may ask whether it is fair to punish one eternally for temporal sins. But would it be fair to massacred children to have their murderers share the vision of God with them? It would, on the contrary, seem better to not let those who freely chose evil view the blessedness reserved for those they wronged, but rather to be exiled from it and suffer when they see what they have lost, for “there will be wailing and grinding of teeth when you see Abraham, Isaac, and Jacob and all the prophets in the kingdom of God and you yourselves cast out” (Luke 13:28). Yet God will be merciful to all, and even the devil will have some good in hell.
So if God allows evil in order to bring good out of it, then those who commit evil will be punished with a (merciful) justice. But why does the evil need to be committed by demons or men in the first place? Could it not just be natural evil, like an earthquake? For not all evils are committed by demons or men, but rather the nothingness in the (created) demons is also in other parts of creation: in the gospels Christ does not extract demons from all whom He heals, but only some, showing that not all ills are from demons (Luke 4:40-41). So it seems even better if God only allowed all evils to happen by natural events and created us so that we could not even choose evil – for in heaven, we are perfected so that we cannot choose evil. On the other hand, some goods can only be produced from willful evil, e.g., forgiveness and repentance (one cannot forgive an earthquake, nor can it repent).
In summary, then, God decided to allow evil in the world so as to produce good out of it. For some goods can only exist if some evil exists, willful evil above all. Without the tears of the crucifixion, for example, Christ’s disciples would not have had the more-than-surpassing joy of the resurrection. “Those who sow in tears will reap in joy” (Psalm 125(126):5), and the joy will be far sweeter by having followed tears. But given that God allowed evil to pervade the universe for this good, he also had to be just to those who did evil and give them (mercifully) what they deserved for their free choice.