The Supreme Court yesterday struck down the death sentence of a 16-year-old boy guilty of the unprovoked shotgun murder of a policeman who stopped him for a traffic offense.

The court ruled 5 to 4 that the sentencing judge failed to consider whether the boy should be spared the death penalty because of his "turbulent childhood."

The ruling dramatically underscored the court's insistence that a judge or jury, before imposing the ultimate punishment for even the most brutal crime, must consider any detail of a defendant's life or circumstances--his "uniqueness as an individual"--that could justify a lesser sentence.

In addition, capital punishment experts interpreted the ruling as a significant boost to arguments in all death penalty cases that a defendant should be spared because of mental or family conditions prior to the crime. They were disappointed that the court did not address the original major issue of the case: whether people under 18 can receive the death penalty.

The ruling also illustrated why, to the great displeasure of many capital punishment advocates, it has become so difficult to execute anyone. While upholding the constitutionality of the penalty, the court has imposed stricter guidelines on it than ever before in U.S. history, and every new court ruling is seized upon by death-row convicts as grounds for reconsidering their sentence. Out of nearly 900 people put on death row in recent years, only four have been executed. Only one of the four actively fought the punishment.

The sentence for Monty Lee Eddings, now 21, must be reconsidered by the lower courts taking into account his childhood. He can still receive the death penalty under yesterday's ruling.

Joining Justice Lewis F. Powell Jr. in the ruling were Justices William J. Brennan Jr., Thurgood Marshall, John Paul Stevens and, in her first capital punishment test, Sandra Day O'Connor. She wrote a separate concurrence indicating that, like her predecessor, Potter Stewart, she believes in extraordinary procedural safeguards for capital defendants.

Chief Justice Warren E. Burger, joined by Justices Byron White, Harry A. Blackmun and William H. Rehnquist, dissented. "There comes a time in every case when a court must 'bite the bullet,' " the dissenters said. They disputed the interpretation of the majority that the sentencing judge had not considered Eddings' childhood. They said they believed he had considered it and had rejected childhood factors as a mitigating circumstance. They also accused the majority of intruding into the sentencing authority of the state courts.

It was to avoid intruding in that authority that the court last week upheld a 40-year prison sentence for a Virginia man convicted of sale and distribution of nine ounces of marijuana. The death penalty, however, demands tighter procedures and stricter federal court supervision than any other kind of sentence, according to prior Supreme Court holdings.

Eddings and some friends had taken off from his Missouri home "without destination or purpose" in April, 1977, Powell wrote. In the car were a .410-gauge sawed-off shotgun and several rifles Eddings had taken from his father. While driving in Oklahoma, Eddings lost control of the car and was stopped by an Oklahoma Highway Patrol officer.

According to court records, Eddings told his companions he was going to "blow him away." He then loaded the shotgun and fired at the officer, hitting him in the chest.

Eddings pleaded no contest to the murder charge. During the sentencing phase, his lawyer introduced evidence of Eddings' upbringing: a strict disciplinarian father who resorted to excessive physical punishment; a suggestion that his mother was an alcoholic and possibly a prostitute, and a statement that Eddings was emotionally disturbed and that his mental and emotional development was at a level several years below his age.

The sentencing judge, however, said that under the law he would not consider that information in deciding the penalty, though he did take Eddings' age into account.

It was here, Powell wrote yesterday in Eddings vs. Oklahoma, that the judge went wrong. Powell cited the court's 1978 holding, Lockett vs. Ohio, that state death penalty laws must allow for consideration of all mitigating circumstances in sentencing. The same thing applies to judges, Powell wrote.

The system of capital punishment must be "at once consistent and principled," he wrote, "but also humane and sensible to the uniqueness of the individual."

"There can be no doubt that evidence of a turbulent family history, of beatings by a harsh father and of severe emotional disturbance is particularly relevant" in Eddings' case, Powell wrote. "Eddings was not a normal 16-year-old; he had been deprived of the care, concern and paternal attention that children deserve.

" . . . All of this does not suggest an absence of responsibility for the crime of murder, deliberately committed in this case," Powell wrote. "Rather, it is to say that just as the chronological age of a minor is itself a relevant mitigating factor of great weight, so must the background and mental and emotional development of a youthful defendant be duly considered in sentencing."

Powell said that lower courts may determine the weight given such information, "but they may not give it no weight by excluding such evidence from their consideration."