Objection 1. It would seem that the whole goodness and malice of the external action depend on the goodness of the will. For it is written (Matthew 7:18): "A good tree cannot bring forth evil fruit, neither can an evil tree bring forth good fruit." But, according to the gloss, the tree signifies the will, and fruit signifies works. Therefore, it is impossible for the interior act of the will to be good, and the external action evil, or vice versa.
Objection 2. Further, Augustine says (Retract. i, 9) that there is no sin without the will. If therefore there is no sin in the will, there will be none in the external action. And so the whole goodness or malice of the external action depends on the will.
Objection 3. Further, the good and evil of which we are speaking now are differences of the moral act. Now differences make an essential division in a genus, according to the Philosopher (Metaph. vii, 12). Since therefore an act is moral from being voluntary, it seems that goodness and malice in an act are derived from the will alone.
On the contrary, Augustine says (Contra Mendac. vii), that "there are some actions which neither a good end nor a good will can make good."
I answer that, As stated above (Article 1), we may consider a twofold goodness or malice in the external action: one in respect of due matter and circumstances; the other in respect of the order to the end. And that which is in respect of the order to the end, depends entirely on the will: while that which is in respect of due matter or circumstances, depends on the reason: and on this goodness depends the goodness of the will, in so far as the will tends towards it.
Now it must be observed, as was noted above (19, 6, ad 1), that for a thing to be evil, one single defect suffices, whereas, for it to be good simply, it is not enough for it to be good in one point only, it must be good in every respect. If therefore the will be good, both from its proper object and from its end, if follows that the external action is good. But if the will be good from its intention of the end, this is not enough to make the external action good: and if the will be evil either by reason of its intention of the end, or by reason of the act willed, it follows that the external action is evil.
Reply to Objection 1. If the good tree be taken to signify the good will, it must be in so far as the will derives goodness from the act willed and from the end intended.
Reply to Objection 2. A man sins by his will, not only when he wills an evil end; but also when he wills an evil act.
Reply to Objection 3. Voluntariness applies not only to the interior act of the will, but also to external actions, inasmuch as they proceed from the will and the reason. Consequently the difference of good and evil is applicable to both the interior and external act.
Source: Summa Theologica (New Advent)