(CG 3/3). That every Agent acts to some Good (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

What follows from an action different from what was intended by the agent, manifestly happens beside his intention. But evil is different from good, which every agent intends. Therefore evil happens beside the intention.

2. Failure in effect and action follows upon some defect in the principles of action, as a halting gait follows upon crookedness of legs. Now an agent acts by whatever of active power he has, not by what defect of active power he suffers; and according as he acts, so does he intend his end. He intends therefore an end answering to his power. Anything therefore that ensues answering to defect of power will be beside the intention of the agent. But such is evil.

4. In agents that act by intellect, or by any sort of judgement, intention follows apprehension: for intention tends to that which is apprehended as an end. If then experience lights upon something not of the species apprehended, the event will be beside the intention: thus if one intends to eat honey, and eats gall, taking it for honey, that will be beside the intention. But every intellectual agent intends a thing according as he takes it for something good: if then it turns out not to be good but evil, that will be beside the intention.