The plan was to drive from the District to Arlington County and rob someone to recoup the money that the cousins had lost in a dice game that night, prosecutors said.

Carl Diener — bespectacled, older and walking alone in the dark — might have seemed an easy mark. But the assailants did not anticipate Diener’s strength, physical fitness or determination to defend himself, Molly Newton, chief deputy commonwealth’s attorney, said during opening statements in the trial of one of the accused in Arlington Circuit Court.

Diener, 57, fought so hard after Roger Clark III jumped him, Newton said, that Clark’s cousin, Javon Martin, jumped out of a car to intervene and fatally stabbed Diener.

“It was supposed to be an easy grab-and-go,” Newton said. Martin, 25, is on trial and accused of first-degree murder in the December 2009 killing.

Martin’s lawyers say he didn’t do it, and that Clark, 21, whose DNA was found in Diener’s pockets, implicated his cousin to get a reduced sentence.

Carl Diener (Courtesy of Arlington police )

Clark pleaded guilty to first-degree murder in the case a year ago and is awaiting sentencing. Prosecutors offered Clark a reduced sentence if he testified against his cousin.

Martin’s lawyer, Nader Hasan, said that Clark may have stabbed Diener. Prosecutors’ primary evidence against Martin, Hasan said, is Clark’s testimony.

“There’s a clear motivation for Roger Clark to lie to police,” Hasan said.

The cousins lived in the Northwest Washington home of Clark’s father, a District lawyer. They were out that night drinking and gambling with friends, Newton said.

Diener awoke early to get to his job opening Arlington Sport and Health Club a mile from his Lyon Village home. Diener did not own a car or cellphone and walked to work each day before sunrise. People who knew Diener said he did not fear for his safety during those walks, in part because Arlington has relatively little violent crime.

After the stabbing, which occurred in the 3200 block of North 13th Street, Clark went through Diener’s pockets and Martin grabbed a gym bag, Newton said.

When they returned home, the cousins went through the bag and found $50, she said.

“Carl Diener died for $50,” Newton said.

It took Arlington police a year and a half to connect the DNA in Diener’s pocket to Clark. Once they did, Clark was arrested and Martin was charged days later.

Diener was a neighborhood fixture, a gregarious man who exercised regularly and enjoyed a circle of friends at the gym. He had worked at the gym for about 10 years, taking an early shift so he could keep his government job, from which he retired about a year before he was killed. He was a former D.C. health inspector and also worked for the General Services Administration.

Lyon Village Park now has a bench and tennis backboard that are inscribed in Diener’s memory. The backboard was paid for with funds raised by a Girl Scout who lived two blocks from where Diener was killed.

Diener’s sister, Patti Diener Lough, said she was steeling herself for the trial this week.

“I’m dreading this because I have to relive it,” she said. “I have to live through all the gory details of it.”