Objection 1. It would seem that one and the same external action can be both good and evil. For "movement, if continuous, is one and the same" (Phys. v, 4). But one continuous movement can be both good and bad: for instance, a man may go to church continuously, intending at first vainglory, and afterwards the service of God. Therefore one and the same action can be both good and bad.
Objection 2. Further, according to the Philosopher (Phys. iii, 3), action and passion are one act. But the passion may be good, as Christ's was; and the action evil, as that of the Jews. Therefore one and the same act can be both good and evil.
Objection 3. Further, since a servant is an instrument, as it were, of his master, the servant's action is his master's, just as the action of a tool is the workman's action. But it may happen that the servant's action result from his master's good will, and is therefore good: and from the evil will of the servant, and is therefore evil. Therefore the same action can be both good and evil.
On the contrary, The same thing cannot be the subject of contraries. But good and evil are contraries. Therefore the same action cannot be both good and evil.
On the contrary, The same thing cannot be the subject of contraries. But good and evil are contraries. Therefore the same action cannot be both good and evil.
I answer that, Nothing hinders a thing from being one, in so far as it is in one genus, and manifold, in so far as it is referred to another genus. Thus a continuous surface is one, considered as in the genus of quantity; and yet it is manifold, considered as to the genus of color, if it be partly white, and partly black. And accordingly, nothing hinders an action from being one, considered in the natural order; whereas it is not one, considered in the moral order; and vice versa, as we have stated above (3, ad 1; 18, 7, ad 1). For continuous walking is one action, considered in the natural order: but it may resolve itself into many actions, considered in the moral order, if a change take place in the walker's will, for the will is the principle of moral actions. If therefore we consider one action in the moral order, it is impossible for it to be morally both good and evil. Whereas if it be one as to natural and not moral unity, it can be both good and evil.
Reply to Objection 1. This continual movement which proceeds from various intentions, although it is one in the natural order, is not one in the point of moral unity.
Reply to Objection 2. Action and passion belong to the moral order, in so far as they are voluntary. And therefore in so far as they are voluntary in respect of wills that differ, they are two distinct things, and good can be in one of them while evil is in the other.
Reply to Objection 3. The action of the servant, in so far as it proceeds from the will of the servant, is not the master's action: but only in so far as it proceeds from the master's command. Wherefore the evil will of the servant does not make the action evil in this respect.
Source: Summa Theologica (New Advent)