Objection 1. It would seem that the supreme good, God, is the cause of evil. For it is said (Isaiah 45:5-7): "I am the Lord, and there is no other God, forming the light, and creating darkness, making peace, and creating evil." And Amos 3:6, "Shall there be evil in a city, which the Lord hath not done?"
Objection 2. Further, the effect of the secondary cause is reduced to the first cause. But good is the cause of evil, as was said above (Article 1). Therefore, since God is the cause of every good, as was shown above (2, 3; 6, 1,4), it follows that also every evil is from God.
Objection 3. Further, as is said by the Philosopher (Phys. ii, text 30), the cause of both safety and danger of the ship is the same. But God is the cause of the safety of all things. Therefore He is the cause of all perdition and of all evil.
On the contrary, Augustine says (QQ. 83, qu. 21), that, "God is not the author of evil because He is not the cause of tending to not-being."
I answer that, As appears from what was said (1), the evil which consists in the defect of action is always caused by the defect of the agent. But in God there is no defect, but the highest perfection, as was shown above (Question 4, Article 1). Hence, the evil which consists in defect of action, or which is caused by defect of the agent, is not reduced to God as to its cause.
But the evil which consists in the corruption of some things is reduced to God as the cause. And this appears as regards both natural things and voluntary things. For it was said (1) that some agent inasmuch as it produces by its power a form to which follows corruption and defect, causes by its power that corruption and defect. But it is manifest that the form which God chiefly intends in things created is the good of the order of the universe. Now, the order of the universe requires, as was said above (22, 2, ad 2; 48, 2), that there should be some things that can, and do sometimes, fail. And thus God, by causing in things the good of the order of the universe, consequently and as it were by accident, causes the corruptions of things, according to 1 Samuel 2:6: "The Lord killeth and maketh alive." But when we read that "God hath not made death" (Wisdom 1:13), the sense is that God does not will death for its own sake. Nevertheless the order of justice belongs to the order of the universe; and this requires that penalty should be dealt out to sinners. And so God is the author of the evil which is penalty, but not of the evil which is fault, by reason of what is said above.
Reply to Objection 1. These passages refer to the evil of penalty, and not to the evil of fault.
Reply to Objection 2. The effect of the deficient secondary cause is reduced to the first non-deficient cause as regards what it has of being and perfection, but not as regards what it has of defect; just as whatever there is of motion in the act of limping is caused by the motive power, whereas what there is of obliqueness in it does not come from the motive power, but from the curvature of the leg. And, likewise, whatever there is of being and action in a bad action, is reduced to God as the cause; whereas whatever defect is in it is not caused by God, but by the deficient secondary cause.
Reply to Objection 3. The sinking of a ship is attributed to the sailor as the cause, from the fact that he does not fulfil what the safety of the ship requires; but God does not fail in doing what is necessary for the safety of all. Hence there is no parity.
Source: Summa Theologica (New Advent)