I have always found that mercy bears richer fruits than strict justice.
Running down the origin of this quote was a lot of fun. It sounded like something that Abraham Lincoln would have said, but I had difficulty finding a source for it. It is cited all over the internet, but no reference is given other than a speech in 1865, and such a lack of citation is often the sign of a spurious quote. After some searching I found it. It is sourced in a conversation that Joseph Gillespie had with Abraham Lincoln. Gillespie was a fellow member with Lincoln of the Illinois General Assembly. With Lincoln he helped found the Republican party in Illinois. Elected a circuit court judge in 1861, he helped set up the Illinois Appellate Court.
During a visit to Washington in Spring of 1864, Gillespie met with Lincoln and, among other subjects they discussed, Lincoln mentioned the problem of captured paroled Confederate troops who were found in arms before they had properly been exchanged:
These men are liable to be put to death when recaptured for breach of parole. If we do not do something of that sort, this outrage will be repeated on every occasion…It is indeed a serious question, and I have been more sorely tried by it than any other that has occurred during the war. It will be an act of great injustice to our soldiers to allow the paroled rebels to be put into the field without exchange. Such a practice would demoralize almost any army in the world if played off upon them. It would be nearly impossible to induce them to spare the lives of prisoners they might capture. On the other hand, these men were no doubt told by their superiors that they had been exchanged and it would be hard to put them to death under any circumstances. On the whole, my impression is that mercy bears richer fruits than any other attribute.