Joseph N. Martin, a 28-year-old former life insurance salesman, was portrayed by a prosecutor yesterday as a man "motivated by simple human greed . . . who had two people brutally killed simply for money" as his murder-conspiracy trial began in Arlington Circuit Court.

In a five-hour opening statement to the jury, special prosecutor Donald S.

Caruthers Jr. charged that Martin hired Richard Lee Earmand to arrange one of the most notorious double murders in Northern Virginia. Real estate agent Alan Foreman and his fiance, Donna Shoemaker, were found shot to death in the garage of their north Arlington home in May 1977.

The deaths were the culmination of a complex insurance scheme designed by Martin to "finance a flashy life style way beyond his income," Caruthers said.

The prosecutor questioned how Martin could reconcile his $12,000 annual salary and relatively small bonusus with his ownership of an expensive home in Sterling Park and two cars, one a new powder blue Mercedes-Benz.

Caruthers charged yesterday that Martin, formerly an agent for New York Life Insurance, sold Foreman a $56,000 life-insurance policy and named a third man as a beneficiary. Prosecutors allege that Martin had Freman killed in order to collect on that policy.

Earman, 36, a former real estate salesman, is in jail awaiting sentencing on a conspiracy charge in connection with deaths. Under terms of his plea agreement he is expected to testify against Martin.

Defense attorne Gerard F. Treanor acknowledged yesterday that Martin, who maintains he is innocent, "perhaps spent money . . . in a manner that stretched his income" and "defrauded his employer" through various schemes. "But that is a log way . . . from the brutal, cold-blooded, vicious murders" of Foreman and Shoemaker, Treanor said.

"Mr. Earman is the one critical witness in this case," Treanor said. "Unless you believe the testimony of Mr. Earman . . . the complexly-built structure erected by Mr. Caruthers will collapse like a house of cards."

The trial is scheduled to continue Monday and is expected to last six weeks.

Both Earman and Martin were brought to trial on murder charges in 1977, but the case against Martin was dropped in the middle of the trail and Earman was acquitted of the murders. He has since testified that he killed the couple at Martin's request.