Is God the Cause of Human Sin?

Summa Theologiae, I, Q49, a1 and a2; I-II, Q9, a6; I-II, Q75, a3; I-II Q79, a1
 
In Part I, Q49, a1, Thomas explains that evil has no per se efficient cause, but it does always have a per accidens efficient cause. Every evil act or situation is good insofar as it exists, and something must cause per se that element of goodness. However, there need be no per se cause of the deficiency that is evil in the thing. When something good causes something bad, there is always some pre-existing deficiency of some kind, either in the agent or the patient, or both.
 
Since there is no defect in God, when God causes something that is evil, there must be some pre-existing defect in the patient. As Thomas puts it in reply 2, “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.”
 
When Thomas explains, in I-II Q9, a6, how God moves the human will, he points to two ways. First, God causes our nature, which includes the free will and its natural inclinations. And, second, God is absolute goodness—so every apparently good thing that moves the will does so because of its apparent participation in God’s goodness. In neither of these ways is God responsible for our sin.
 
In I-II Q75, a3, Thomas insists that nothing external to the will can necessitate sin. God is the cause of the movement of the will that is sin, but God (in His essence) does not necessitate that the will choose badly.
 
In I-II Q79, a1, “Whether God is a cause of sin,” Thomas summarizes his conclusions:
 
“Now God cannot be directly the cause of sin, either in Himself or in another, since every sin is a departure from the order which is to God as the end: whereas God inclines and turns all things to Himself as to their last end, as Dionysius states (Div. Nom. i): so that it is impossible that He should be either to Himself or to another the cause of departing from the order which is to Himself. Therefore, He cannot be directly the cause of sin.”