The jury filed out of a packed courtroom late today to choose a foreman, then returned minutes later and deliberated for 2 1/2 hours in the double-murder trial of Wayne B. Williams. The jurors, who are sequestered, then recessed for the night.

In final arguments today prosecutors portrayed Williams as a "mad-dog killer" who preyed on young ghetto blacks and terrorized this city for two years.

"He's cool, cunning, evil," Fulton County District Attorney Lewis Slaton said. "You will have to be responsible for putting him back in the community."

One of Williams' lawyers, Mary Welcome, painted him as the victim of desperate police seeking to capture a killer at all costs and begged the jury of eight blacks and four whites "not to put the seal of approval on the execution of a scapegoat."

Williams, 23, stared straight ahead, hands folded in his lap. At one point, apparently on the verge of tears, he took off his glasses and rubbed his eyes.

Later he turned and glared at Atlanta Police Commissioner Lee Brown, who sat alongside top police officials responsible for his arrest and the investigation of the slayings of 28 young blacks over a 22-month period here.

Williams is accused of killing two of the victims, Nathaniel Cater, 27, and Jimmy Ray Payne, 21, and throwing their bodies into the Chattahochee River. But prosecutors were allowed to introduce evidence gathered in the investigation of 10 other killings in an attempt to show "pattern, scheme and bent of mind."

Today, for the first time in the complex, nine-week trial, the prosecution described Williams as the "repeat killer" who held this city in the grip of fear for 22 months, taking delight in baffling police with a trail of bodies and few clues.

Williams fit a profile of the killer, deputy prosecutor Jack Mallard said, because his job as a free-lance television cameraman cruising the city and as a self-styled music promoter enabled him to blend into the street scene.

Mallard, who cracked Williams' cool persona on the witness stand earlier this week, accused him of stripping his victims and throwing them into rivers to wash off evidence after newspapers described how police had found fibers on the bodies.

"Lo and behold," Mallard said, "the mad-dog killer seizes on it. He's smart. After he reads about it, six bodies are fished out of rivers in a few months. You can't tell me those aren't connected."

Prosecutors reviewed the circumstantial evidence against Williams:

* More than 700 fibers, taken from 12 victims and matching fibers taken from materials in the Williams' home and station wagon.

* Witnesses who placed him with Cater, Payne and several other victims.

* Contradictory accounts of what happened early last May 22 when he was stopped for questioning after he drove across a bridge above the Chattahoochee River and police heard a splash that they contend was caused by Cater's body hitting the water.

Two days later, the nude body of Cater, an unemployed alcoholic, washed up downstream near the spot where the body of Payne, an ex-convict who yearned to be a singer, had been found a month earlier.

"What makes a person tick who would do these things?" Mallard asked. "Frustration builds up. A storm rages within."

In Williams' case, Mallard said, the storm broke after he lost the childhood radio station that brought him brief notoriety but bankrupted his parents.

He described Williams as a failure who made thousands of demonstration tapes as a music promoter but never "had a succesful deal. He never made enough money to buy a hamburger. His resume is all hype or out-and-out lies."

Prosecutors contend that Williams was so enraged by young black street hustlers that he killed them to purify his race--like "Adolf Hitler," Slaton said. Williams took the bodies to the bridge in his car and hoisted them over the railing, prosecutors said, even though there are no witnesses to the murders.

"How many people testified, 'I saw that man kill somebody?' " defense attorney Alvin Binder asked. "Not one person."

Scoffing at the notion that Williams is strong enough to lift a body, Binder called his client a "pudgy, fat little boy," ordered him to stand before the jury, then slapped Williams' stomach. "There's no muscle there," he said. "And he's not sinister, either."

Binder stood before an exhibit of the Jackson Parkway Bridge across which Williams said he drove early May 22 in search of a singer who has never been located.

The exhibit was propped by pillars representing evidence against his client. One by one he removed them, and the bridge came tumbling down, revealing the words "Reasonable Doubt" written in red ink.

The courtroom is filled with so many exhibits, from blowups of microscopic fibers to two life-sized plastic dummies nicknamed Horace and Ferdinand, that Judge Clarence Cooper is allowing the jury to deliberate there for convenience.

Slaton shored up the pillars in the closing argument. He said no similar murders have occurred since Williams' arrest because "we had him under surveillance or in custody."

Then he called Williams "stupid" for dropping Cater's body off a bridge staked out by police recruits. "The 'Keystone Kops' got him," said Slaton, using one of Williams' description of police who followed him in the weeks before his arrest. "He'd failed again."