What do you think that Abraham Lincoln meant when he said, "The severest justice may not always be the best policy"?
Sometimes people want to get revenge just for the sake of revenge. However, this does not mean that society is really better off. Just punishing someone, or a group, does not really fix anything on its own. Sometimes it only makes things worse.
When I read this quote I immediately thought about Guantamo Bay (some may really disagree with this answer that I'm about to write--).
There are criminals there who have plotted against America and have even participated in terrorist attacks. These people, if anyone, deserve punishment, which would be serving justice, right?
However, some of these people have been subjected to torture so that additional information can be extracted from them. Some would say this is guaranteeing "severe justice"--they are only getting what they deserve.
But, is torture the best policy? No. Sure, these men (and women?) are getting their just deserts, but this is not the best policy because the American use of torture opens up the possibility that our troops will be subjected to torture at the hands of other governments.
Lincoln liked moderation and fairness--even justice, in its must immoderate form, can be dangerous.
Lincoln clearly understood that the same punishment does not fit all. It is not a "one-fits-all" justice in some cases. He realized that there should be exceptions to the rules sometimes, depending upon each situation.
Justice involves punishment for wrongdoing. Severe justice would remove the possibility of that wrongdoing occuring again, in most cases removing the wrongdoer from society. Lincoln did not have that option with the Confederacy, since he had fought a war to keep them in the Union, not separate them from it. To punish with severe justice would embitter a defeated people, which is something that Lincoln did not want. His purpose was to bring them back into the "family" at their original level. Rather than justice, he desired to show mercy, "with malice toward none, with charity for all." However, with his assassination, the Congress chose to inflict severe justice on the South, embittering them with a bitterness that lingers still today, as well as condemning them to decades of poverty.
In this case, the best policy would have been a severe mercy.