A Montgomery County jury found a District man guilty yesterday of robbing and literally scaring to death a motel clerk, then softened its verdict with an unusual plea for mercy in sentencing.

Rejecting a defense of not guilty by reason of insanity, the jurors nonetheless asked that the state do everything in its power for "the psychological and physical rehabilitation" of Michael Stewart, a 20-year-old man who, according to his defense attorneys, was sexually abused as a child and used drugs.

The jury found Stewart guilty of robbery and felony murder charges stemming from the fatal heart attack suffered by Pearl A. Pizzamiglio, a 60-year-old clerk at the In Town Motor Hotel in Chevy Chase. Stewart was charged with entering the motor inn on Nov. 19, as another man stood lookout and two men waited in a getaway car. Stewart handed Pizzamiglio a paper bag with a note telling her to put money in the bag so she would not be hurt, prosecutors said.

Pizzamiglio, who turned over $167, was not physically attacked, but she died two hours later.

Under Maryland law, felony murder is committed when a death results from the commission of another felony such as arson, rape, or robbery. Felony murder convictions have been won in similar cases involving stress-related heart attacks in Arizona, California and New Jersey, according to a spokesman for the state's attorney's office.

Stewart, clad in a jail-issued navy blue shirt and pants and beige terry cloth slippers, was led into the courtroom yesterday to hear the verdict. His defense attorney, Barry Helfand, embraced the tall man and cautioned, "Whatever they say -- guilty, innocent, crazy -- just sit there. Don't say anything. Take it like a man." Stewart, who is heavily medicated with antipsychotic drugs, nodded his agreement, sat down and waved at a reporter.

Stewart could receive maximum penalties of life in prison for the felony murder conviction and 10 years for the robbery conviction. But prosecutor Richard E. Jordan said yesterday the state would not seek the stiffest sentences. He said that Stewart is a "good candidate" for incarceration at Patuxent Institution in Jessup, where emotionally unbalanced felons are treated during their confinement.

Circuit Court Judge Calvin Sanders set Stewart's sentencing for Nov. 23, five days before Stewart's three alleged accomplices, who have pleaded not guilty, are scheduled for trial.

During closing arguments late Monday, Jordan and Helfand condensed for the jury the complex evidence presented at the week-long trial.

Helfand dismissed the death-by-fright theory asa fabrication of the prosecution. He acknowledged that Stewart committed the robbery but said Stewart's three "cohorts" chose him to confront the clerk because of his "borderline intelligence."

"This kid is screwed up. They (his three accomplices) picked him out to walk in unmasked," Helfand said.

Helfand said Stewart was a paranoid schizophrenic by the time he was 16, due to his use of drugs and sexual abuse by his older brother.

"Nobody's going to say that Michael Stewart has had the kind of life any of us would want to have," prosecutor Richard Jordan countered. "But certainly Mrs. Pizzamiglio should not have to pay for the kind of life he's had."

Jordan urged the jury to consider "the potent evidence" of last Thursday's testimony by cardiologist Dr. Robert S. Eliot that Pizzamiglio's heart was "a muscle torn apart by a surge of adrenalin" caused by her terror during the robbery.

Although there was no evidence of a weapon used during the robbery, Jordan said, "The intimidation, the fear instilled in Mrs. Pizzamiglio were just as lethal to her as a bullet through her head or a knife through her heart."