Charles M. Wantland was convicted of premeditated murder and a first-degree sexual offense yesterday for the stabbing death last year of a 12-year-old Clinton boy, Donald Alan Henley.

The dead boy's mother screamed and cried when the Prince George's County jury foreman pronounced the guilty verdicts, which came less than 90 minutes after the prosecutor closed his case by glaring at Wantland and telling the jury the defendant had "a depraved indifference to human life."

The murder and sexual offense convictions carry mandatory life sentences, but the prosecutor, Deputy State's Attorney Joseph Sauerwein, said after the trial: "Wantland deserves the death penalty. He just slipped underneath it."

Henley was killed on June 17, 1978, two weeks before Maryland's new death penalty law took effect.

Wantland, 44, had been paroled three weeks earlier from a state correctional institution where he had been serving a sentence for second-degree murder. He was living and working in a dilapidated Clinton mansion as part of a federally funded and county-supervised job training program when the Henley slaying occurred.

Several jurors said after the trial that "everybody believed he was guilty" almost from the moment they entered the jury room for deliberations. One juror said the only question was whether to find Wantland guilty of first-degree, second-degree or felony murder.

Pictures of the small, blond Henley boy, partically clothed and covered with blood, had made several of the jurors "feel like crying," said juror Agnes Curtis.

"They say you're not supposed to show emotion when you're on a jury, but that's hard in a case like this," Curtis said.

The Henley boy, according to testimony, had encountered Wantland on a Clinton street as Henley was running home in the rain after an outing with a playmate that June day. The boy's body was found by searchers the next day in a wooded area near the old Berger mansion, where Wantland was living.

The prosecution focused its case on three statements police said Wantland made to them after his arrest.

In one, Wantland allegedly told police: "I must have done it. I must have done it . . . I want to die. Take me out back and shoot me," according to Sauerwein's closing arguments.

In another, Wantland was said to have told a second officer that he blacked out on the day of the slaying after taking the boy home with him to see a model boat. Wantland told police he recalled standing by a pond near the mansion at about 4 p.m., but that his next recollection, later that night, was hearing searchers call Henley's name.

Buy defense attorney Joseph Niland argued to the jury that these oral statement's were not Wantland's. They were "reconstructions" made by three police officers - "statements they knew were in the best interest of the case," Niland told the jurors.

"If police wanted to present reliable statements, would they all be coming in here with no written statement, nothing on tape, no videotape . . . no notes?" the defense attorney asked.

In an angry rebuttal, Sauerwein told jurors: "The defense says to disregard these statements . . . to believe three officers came into the courtroom and committed perjury of believe the statements were taken . . . involuntarily. Both charges are completely absurd."

Several jurors said later it was the prosecution's case as a whole that convinced them of Wantland's guilt - the three statements, the circumstantial evidence and several witnesses' identification of Wantland as the man they saw in the Clinton neighborhood the day Henley was killed.

"Those witnesses were so consistent, they had nothing to gain by coming here and lying," said juror Curtis.

Henley's mother, Carol, and his older sister, Linda, who sat through the entire seven-day trial, left the courtroom in tears after hugging and thanking Sauerwein.

The father, Larry Henley, remained stone-faced as the verdicts were read and as he left the courtroom of Circuit Judge Jacob S. Levin.

All three family members silently shook their heads when asked if they had any comment on the case.

But family attorney John Pyne said they are planning to sue the county for "general negligence in supervising" the job training program in which Wantland worked. The county "failed to inform the community that the program was going on, so families could warn their children to be careful," Pyne said.

Judge Levin set Sept. 7 for Wantland's sentencing.

Sauerwein said that a life sentence, which is mandatory on the sex offense and murder charges, allows parole after about 12 years.

"But let's hope they never let him out," Sauerwein said. "They did last time, after five years of a 30-year sentence, and you saw what happened." CAPTION: Picture, CHARLES M. WANTLAND . . . faces life sentences