Amnesty International has added capital punishment to its catalogue of crimes against human rights. The logic has escaped some of those who have hitherto supported the organization. In the latest issue of Matchbox, the Amnesty International newsletter, a correspondent writes: ''I have been proud to be a member of Amnesty International for several years. . . . You are doing God's work in trying to prevent the torture and murder of political prisoners. But now you are mixing up a sentimental pot of tripe trying to save sadistic fiends from the penalty they deserve.''
And so the debate on capital punishment revives, just at a moment when we are being reminded that human life is not always held sacred. The massacres in El Salvador, the execution of more than 4,000 Iranians since the revolution (an estimate by Amnesty International), the homicide rate in the United States (one killing every 26 minutes) - these facts humiliate all of us who prefer to think of ourselves as civilized. But we divide in our reactions.
For some, like the Amnesty International correspondent, all the evidence of ''barbarous cruelty,'' as he puts it, seems to inspire a desire to punish to the utmost any individual who can be held responsible.
For others, the reaction is to want to stop the chain of killing - of murder and retribution - at the place where it can be stopped: capital punishment.
Most European countries have revoked capital punishment, but the American debate goes on, making it one of our oldest issues. In the spring of 1787, even before the constitutional convention, Dr. Benjamin Rush of Philadelphia read a paper to a roomful of guests in Ben Franklin's house, opposing capital punishment.
The basic arguments pro and con have changed remarkably little. The defenders of capital punishment single out the cases of the ''uncontrolled brutes,'' in the words of one advocate. The argument is that, in pitying the victim of capital punishment, one tends to forget his victims. Where was ''sanctity of life'' then?
The proponents of abolition single out the cases of innocent men wrongly executed. Furthermore, today's abolitionists, like Yale law professor Charles L. Black, point out: ''Most people on death row are black, and almost all are poor.''
The defenders say the problem is to find a humane method of execution. The abolitionists say murder is murder, and if the first murder is wrong, can any form of capital punishment - ''legal homicide'' - be right? The phrase belongs to Arthur Koestler, who spent three months of 1937 under sentence of death in Spain. In the heat of civil war, the novelist-journalist was suspected of being a spy.
The pros argue that the death penalty is a deterrent - J. Edgar Hoover once compared it rather puzzlingly to the warning beam from a lighthouse. The cons produce studies that disagree.
And so it goes, rather endlessly, with public sentiment swinging back and forth. According to polls, 68 percent of Americans favored keeping capital punishment in 1953. By 1966 that figure had dropped to 38 percent. But polls today suggest that attitudes are toughening again.
We are appalled by the violence we perceive about us - armies amok, ideological terrorists everywhere, hometown madmen on our own dark streets, killing for no reason we can, or dare to, understand. There is an outrage in the face of all this rage.
But always a pendulum seems to swing within us, individually as well as collectively. We recognize, if reluctantly, that war and capital punishment are the two acts of killing in which all citizens must hold themselves accountable. As more than one soldier, and more than one executioner, have told us: They are doing our dirty work. This thought can appall us almost as much as the violence we might hope to defeat.
Nuclear war, capital punishment, abortion - all three life-and-death issues seem to obsess us. Though we may take different positions on all three, though we recognize how far they are from being equivalent, we sense that they plumb to the depths of our secret confusions. Like few other issues, they expose us to our hopes and fears about human existence, and about ourselves. They are our appointed questions these days, and will not soon go away.
''Thou shalt not kill'' - it is a simple commandment to which most people subscribe, more perhaps than to any other. Why - we must ask ourselves in some frustration after all these years - does it still lead to differences of opinion that can only be described as violent?