If inquiry shows that Colonel Muammar Gaddafi and his son Mutassim were summarily executed last Thursday, theirs will be the latest in a series of high-profile killings this year, beginning with Osama bin Laden in Pakistan and continuing with Anwar al-Awlaki and his bomb-making colleague Ibrahim al-Asiri in Yemen.

No one can doubt that the quick dispatch of these men was convenient. If Bin Laden had been taken to Guantanamo or The Hague and put on trial, his status would have risen again, and many more deaths might have occurred in reaction to his humiliation at the hands of America. Killing him and quickly disposing of his remains at sea had a strong, utilitarian, short-term justification.

But for the long-term, it is a very bad thing that he was assassinated rather than put on trial. The same applies to Awlaki, and it applies also, this time in the interests of Libya's future social health, to Gaddafi. The idea of the rule of law, of its due process, of the civil liberties accorded even to those we know are guilty of vile and violent deeds – indeed, the whole project of civilising the world and substituting peace and law for violence and revenge – is jeopardised by using murder rather than law to deal with such criminals.

When leading nations such as the United States act like Mexican drug gangsters, they harm themselves and set a terrible example to the world. When a nation freeing itself from tyranny uses a tyrant's methods, it gives itself a steeper hill to climb towards becoming a just and stable society.

In accepting the pragmatic case for shooting malefactors, just as we shoot mad dogs, we state that we do not wish to pay the high cost of living according to law and civil liberties. We champion our Western principles about the rule of law and the rights of individuals, we thus say, only until they become a burden and an inconvenience; and, when they do, we summarily shoot people in the head instead. In effect, we admit the shameful fact that these principles are mere pieties that we do not really believe in, because we ditch them when occasion demands. And in this way we are no different from the Gaddafis and Bin Ladens.

President Obama's administration has tried to find legal sanction for the killing of Bin Laden and Awlaki, but that misses the point. It is in the fundamental long-term interests of all of us that we should accept the shorter-term difficulties that arise from sticking to principle. This series of summary killings has damaged civilised values and set progress back, you might say, to medieval ways.

The Libyan people have shown great courage in combating a dictator who still had much support, including an army. It would have been an act of even greater courage to put Gaddafi on trial. That would have demonstrated an intention to behave far better than he and his sons did, and to build a far better and more just society.