Objection 1. It would seem that a human action is not meritorious or demeritorious on account of its goodness or malice. For we speak of merit or demerit in relation to retribution, which has no place save in matters relating to another person. But good or evil actions are not all related to another person, for some are related to the person of the agent. Therefore not every good or evil human action is meritorious or demeritorious.
Objection 2. Further, no one deserves punishment or reward for doing as he chooses with that of which he is master: thus if a man destroys what belongs to him, he is not punished, as if he had destroyed what belongs to another. But man is master of his own actions. Therefore a man does not merit punishment or reward, through putting his action to a good or evil purpose.
Objection 3. Further, if a man acquire some good for himself, he does not on that account deserve to be benefited by another man: and the same applies to evil. Now a good action is itself a kind of good and perfection of the agent: while an inordinate action is his evil. Therefore a man does not merit or demerit, from the fact that he does a good or an evil deed.
On the contrary, It is written (Isaiah 3:10-11): "Say to the just man that it is well; for he shall eat the fruit of his doings. Woe to the wicked unto evil; for the reward of his hands shall be given him."
I answer that, We speak of merit and demerit, in relation to retribution, rendered according to justice. Now, retribution according to justice is rendered to a man, by reason of his having done something to another's advantage or hurt. It must, moreover, be observed that every individual member of a society is, in a fashion, a part and member of the whole society. Wherefore, any good or evil, done to the member of a society, redounds on the whole society: thus, who hurts the hand, hurts the man. When, therefore, anyone does good or evil to another individual, there is a twofold measure of merit or demerit in his action: first, in respect of the retribution owed to him by the individual to whom he has done good or harm; secondly, in respect of the retribution owed to him by the whole of society. Now when a man ordains his action directly for the good or evil of the whole society, retribution is owed to him, before and above all, by the whole society; secondarily, by all the parts of society. Whereas when a man does that which conduces to his own benefit or disadvantage, then again is retribution owed to him, in so far as this too affects the community, forasmuch as he is a part of society: although retribution is not due to him, in so far as it conduces to the good or harm of an individual, who is identical with the agent: unless, perchance, he owe retribution to himself, by a sort of resemblance, in so far as man is said to be just to himself.
It is therefore evident that a good or evil action deserves praise or blame, in so far as it is in the power of the will: that it is right or sinful, according as it is ordained to the end; and that its merit or demerit depends on the recompense for justice or injustice towards another.
Reply to Objection 1. A man's good or evil actions, although not ordained to the good or evil of another individual, are nevertheless ordained to the good or evil of another, i.e. the community.
Reply to Objection 2. Man is master of his actions; and yet, in so far as he belongs to another, i.e. the community, of which he forms part, he merits or demerits, inasmuch as he disposes his actions well or ill: just as if he were to dispense well or ill other belongings of his, in respect of which he is bound to serve the community.
Reply to Objection 3. This very good or evil, which a man does to himself by his action, redounds to the community, as stated above.
Source: Summa Theologica (New Advent)