Joe Giarratano may have killed Barbara Kline and her daughter on a February night in 1979. Then again, he may not have killed them. But now the commonwealth of Virginia is preparing to kill Joe, and that prospect gives me pause.

The issue of capital punishment strongly, emotionally, divides the American people. One school of thought follows Justices William Brennan and Thurgood Marshall. They believe that a death sentence never can be justified, no matter the circumstances.Another school holds that certain crimes are so vile, and certain criminals so evil, that death sentences are fully warranted. I am of that school.

But surely both schools of thought are agreed on one point: If a convicted defendant is to be put to death, we must be absolutely certain - certain beyond the faintest shadow of a reasonable doubt - that the defendant is guilty. I have read the record in the case of Joe Giarratano, and I don't know. I simply don't know. But I can't see that any useful public purpose would be served by sending him to the chair.

The facts are not greatly in dispute. It was a sordid case, but our society has its sordid aspects. Joe was then 22, the product of a wretched boyhood. He was on drugs at 11; he first tried to commit suicide at 15. In the winter of 1978-79, he was working on a scallop boat operating out of the Norfolk area. When he was not working he was drunk or drugged.

During most of January he lived in a house with Barbara Kline and her 15-year-old daughter, Michelle. He had sexual relations with both of them. Around Feb. 1 he moved out, but he returned to the house on the night of Feb. 4.

At this point the facts get fuzzy. Joe's story is that he awoke in a stupor of drugs and alcohol, saw the two bodies and fled by bus to Jacksonville, Fla. He saw a deputy sheriff in the Greyhound station. Joe walked up to him, said he had killed two women and wanted to turn himself in. The officer read him his Miranda rights and put him under arrest. There was a spot of blood on his right shoe.

During the chaotic weeks that followed, Giarratano twice attempted suicide. He gave four signed confessions. He went through a state mental examination and was found not insane. Insisting on his guilt, he waived jury trial; he was found guilty in May and sentenced to death in August. The Virginia Supreme Court affirmed the conviction. Further legal proceedings are still in progress.

During his first four years on death row, Giarratano was a mental basket case. Then expert counseling began to work, and he became a model prisoner; he studied law so effectively that one aspect of his case is now before the U.S. Supreme Court (Murray v. Giarratano, argued in March, testing the right of death row defendants to have an experienced lawyer provided for post-conviction appeals).

In recent years, Joe's friends on the outside have launched a desperate effort to gain a new trial. The only probative evidence against him in 1979 were the four confessions. Without these the commonwealth had no case. The confessions were sharply contradictory in important respects, suggesting that the accused yielded to police persuasion. ("Is this the way it could have happened, Joe?")

The blood spot on his shoe matched the blood type of the daughter; it never was matched with her mother's. Joe insisted that he strangled the girl with his bare hands; medical evidence was to the contrary. The older woman was bloodily stabbed to death; photographs of the crime scene showed bloody footprints, but a forensic serologist found no evidence of blood on the soles of his boots. New evidence indicates that Barbara Kline was stabbed by a right-handed assailant; Joe is left-handed and has a neurological weakness in his right hand. And so on.

In recent weeks the newspapers have told us of Jim Richardson, sentenced to death in Florida for killing his children. As a result of evidence discovered after his trial, Richardson has been set free. Joe is on death row. I am a neutral observer, not known to be soft on crime, and I am filled with reasonable doubt.