II.2 chapter 3

So how can there be that are called enemies of God?
They withstand the commandement of God, but do not harm God by doing that, but themselves (because God is unchangeable and incorruptable). Their vice is contrary to their nature, and therefore contrary to the good and thus contrary to God (which is the supreme good); but the vice doesn't harm God like it harms their nature. For to God no evils are harmful; they only harm the changeable and corruptable natures.

Something evil is always something harmful. Indeed the concept 'evil' means that it is contrary to something good. 'Evil' is no concept for something substantial, but for something depriving something of good, something that therefore has good nature.

But does the punishment of someone vicious do anything good to their existence?
Because that punishment is just, it is good. It is just because every such vice originates in the will, for even the vice which by the force of habit and long continuance has become a second nature, had its origin in the will (for we are speaking of natures that have a mental capacity for that enlightenment which discriminates between what is just and what is unjust).