Paul E. Wersick set out to rob somebody last July 10 because he felt "angry, frustrated and degraded" after a fight with his mother, and he wanted "to make somebody else feel as bad as he did," a psychiatrist testified yesterday at the 16-year-old's murder trial.

But after Wersick started to rob Montgomery County tax assessor George Angerman Jr., the youth lapsed into a "transitory psychotic state" and was legally insane when he fatally shot Angerman, according to Dr. Reginald S. Lourie, who was testifying on Wersick's behalf.

Angerman, who had been shot once in the head, was found July 11 slumped in the front seat of his car in a Rockville underground garage, his mouth gagged with a red boy scout neckerchief and his left wrist tied with a piece of nylon rope.

Yesterday, on the fourth day of Wersick's trial in Montgomery Circuit Court in Rockville, Lourie testified that Wersick, then 15, was acting on the impulses of 3-year-old when he set out to rob somebody, to "degrade" them.

"The degradation as he planned it and wanted to carry it out was robbery," Lourie stated under cross-examination. "It was in the process of that act - a robbery - that the shooting took place."

Wersick, he testified, was mentally "out of control" for only a few moments, "probably a matter of seconds."

During such a psychotic state, Wersick was "not in control of his thinking and acting," the psychiatrist testified, and he was "not responsible" for his acts.

Asked by the presiding trial judge whether any "act occurred" to cause the shooting, Louire said, "As far as I know there was no act on the part of the man he had robbed that would have precipitated this."

Lourie's testimony came yesterday after prosecutors rested their muder case against Wersick, and Public Defender J. James McKenna sought to introduce testimony to support the defense contention that Wersick is innocent by reason of insanity.

Because all defendants are presumed sane under Maryland law, McKenna first had to show the presiding judge that there was sufficient reason to put the sanity question before the 12 jurors in the case. After a two-hour hearing, Judge Philip M. Fairbanks ruled that the jury could hear the insanity defense.

The jury was scheduled to return to court to hear the testimony of Lourie and other psychiatrists Friday.

Yesterday, before the prosecution rested, Montgomery County Police Sgt. Paul Smith testified that two fingerprints found on the inside of the driver's window in Angerman's car matched the fingerprints of Wersick. It was that matchup, of fingerprints on the murder scene and those in police files, that led police last July to the short, chunky youth now on trial on charges of murder.

With Smith's testimony, Assistant State's Attorney Larry Ceppos drew to a close the methodical case of technical evidence he had pieced together against Wersick during the first three days of the trial.

What emerged from the prosecution's case was the picture of a teenager with a bizarre fascination with weaponry.

Yesterday, psychiatrist Lourie said under cross-examination that in his diagnosis of Wersick he considered among numerous factors "the episode in which a young girl was killed."

Wersick, at the age of 14 was found by a county juvenile judge to have been "involved" in the shooting death of an 11-year-old playmate. He was put on probation and returned to the Montgomery County school system.

Lourie testified that Wersick functioned at the level of a 3-year-old in many ways. And though his intellect "was superior," he used it to leave situations, and began to "live more and more in fantasy," Lourie said. In school the youth was "baited by other children," he couldn't stand to live in reality, and "even his intellect began to deteriorate," the doctor said.