19 Some man will say, “So then any thief whatever is to be accounted equal with that thief who steals with will of mercy?” Who would say this? But of these two it does not follow that any is good, because one is worse. He is worse who steals through coveting, than he who steals through pity: but if all theft be sin, from all theft we must abstain. For who can say that people may sin, even though one sin be damnable, another venial? But now we are asking, if a man shall do this or that, who will not sin or will sin?
Not, who will sin more heavily or lightly. For even thefts themselves are more lightly punished by law than crimes of lust: they are, however, both sins, albeit the one lighter, the other heavier; so that a theft which is committed of concupiescence is held to be lighter than an act of lust which is committed for doing a good turn. Namely, in their own kind these become lighter than other sins of the same kind, which appear to be committed with a good intention; when yet the same compared with sins of another kind lighter in respect of the kind itself, are found to be heavier.
It is a heavier sin to commit theft of avarice than of mercy; and likewise it is a heavier sin to perpetrate lewdness of luxury, than of mercy; and yet is it a heavier sin to commit adultery of mercy, than to commit theft of avarice. Nor is it our concern now, what is lighter or what heavier, but what are sins or are not. For no man can say that it was a duty for a sin to be done, where it is clearly a sin; but we say that it is a duty, if the sin were done so or so, to forgive or not to forgive.
Source: To Consentius: Against Lying (New Advent)