PRINCETON, N.J. — The recent release of Abdel Basset Ali al-Megrahi, convicted of blowing up Pan Am Flight 103 over Lockerbie, Scotland, in 1988, sparked outrage.

Around the same time, the Philadelphia Eagles, an American football team, offered a second chance to former star Michael Vick, who was convicted of running a dog-fighting operation in which unsuccessful fighters were tortured and killed.

And William Calley, who commanded the platoon that massacred hundreds of Vietnamese civilians at the village of My Lai in 1968, has now broken his media silence and apologized for his actions.

When should we forgive or show mercy to wrongdoers? Many societies treat crimes involving cruelty to animals far too lightly, but Vick’s penalty — 23 months in prison — was substantial. In addition to imprisonment, he missed two years of his playing career, and millions of dollars in earnings. If Vick were never to play football again, he would suffer punishment well beyond that imposed by the court.

Vick has expressed remorse. Perhaps more importantly, he has turned words into deeds, volunteering at an animal shelter and working with the Humane Society of the United States to oppose dog fighting. It is hard to see what good would come from not allowing him to complete his rehabilitation and return to doing what he does best.

Al-Megrahi was convicted of murdering 270 people and sentenced to life imprisonment. He had served only seven years when Kenny MacAskill, the Scottish justice minister, released him on compassionate grounds, based on a medical report that al-Megrahi has terminal cancer and only three months to live.

The question of remorse has not arisen, because Megrahi has never admitted guilt, and did not drop an appeal against his conviction until just before his release.

Doubts have been raised about whether Megrahi is really near death. Only the prison doctor, it seems, was prepared to say that he did not have more than three months to live, while four specialists refused to say how long he might have. There has also been speculation that Megrahi’s release was related to negotiations over oil contracts between Britain and Libya.

But let us leave such questions aside for the moment. Assuming that al-Megrahi was released because he has only a short time to live, does a prisoner’s terminal illness justify compassionate release?

The answer might depend on the nature of the crime, the length of the sentence, and the proportion of it that remains to be served. For a pickpocket who has served half of a two-year sentence, it would be excessively harsh to insist on the sentence being served in full if that meant that he would die in prison, rather than with his family. But to release a man who served only seven years of a life sentence for mass murder is a very different matter. As the victims’ relatives point out, in planning his crime, Megrahi showed no compassion. Why, they ask, should we show compassion to him?

MacAskill, in a statement to the Scottish Parliament defending his decision, refrained from quoting from the best-known speech on mercy in the English language — that of Portia in Shakespeare’s “The Merchant of Venice” — but Portia’s words would have fitted the core of his statement. Portia acknowledges that Shylock is under no obligation to show mercy to Antonio, who is in breach of his agreement to him.

“The quality of mercy is not strained” — that is, constrained, or obligatory — she tells Shylock, but rather something that falls freely, like rain. MacAskill acknowledged that al-Megrahi himself showed no compassion, but rightly points out that this alone is not a reason to deny him compassion in his final days. He then appeals to the values of humanity, compassion, and mercy as “the beliefs we seek to live by” and frames his decision as being true to Scottish values.

We can reasonably disagree with MacAskill’s decision, but we should acknowledge that — unless there is more going on than appears on the surface — he was motivated by some of the finest values we are capable of exercising. And, if we believe that al-Megrahi was not sufficiently punished for his crime, what are we to make of the treatment of former Lt. William Calley?

In 1971, Calley was convicted of the murder of “no less than 22 Vietnamese civilians of undetermined age and sex.” He was also convicted of assault with intent to murder a Vietnamese child. Yet three days after his conviction, President Richard Nixon ordered that he be released from prison and allowed to serve his sentence in a comfortable two-bedroom house. There he lived with a female companion and a staff to assist him. After three years, he was released even from this form of detention.

Calley always claimed that he was following orders. Capt. Ernest Medina, his commanding officer, ordered him to burn the village down and pollute its wells, but there is no clear evidence that the order included killing non-combatants — and of course if such an order were issued, it should not have been obeyed.

After decades of refusing to speak publicly, Calley, who is now 66, recently said that “not a day goes by” when he does not feel remorse “for what happened that day in My Lai.” One wonders if the relatives of those murdered at My Lai are more ready to forgive Calley than the relatives of those killed by at Lockerbie are to forgive al-Megrahi.

Peter Singer is a professor of bioethics at Princeton University. © 2009 Project Syndicate