A doctor killed his best friend more than 30 years ago because he wanted to marry his friend's wife and could see no other way to break up their marriage, a prosecutor told a jury Tuesday.

Though Dr. Stephen Scher and Martin Dillon's wife were already carrying on "an open and notorious affair," Scher wanted more, Senior Deputy Attorney General Patrick Blessington told jurors at the opening of the former allergist's murder trial.

Scher "wanted to take (Dillon's) wife, he wanted to take his kids," Blessington said. "He wanted to replace Marty Dillon."

Scher, now 67 and married to Dillon's widow, is charged with killing Dillon, a 30-year-old lawyer, while the two were skeet shooting in rural northeastern Pennsylvania on June 2, 1976. Scher maintains that Dillon was shot accidentally as the pair struggled over a shotgun.

Scher was convicted in 1997 of first-degree murder and sentenced to life in prison. An appeals court ordered a new trial in 2004.

Scher had claimed for more than 20 years that Dillon tripped and accidentally shot himself while chasing a porcupine. He changed his story in 1997, after prosecution witnesses at his first trial testified that Scher's boots had blood spatter on them and that a tiny piece of Dillon's flesh was found on Scher's pants leg.

Scher's attorney, Joshua Lock, told jurors on Tuesday that if Scher had really wanted to murder Dillon, he would have come up with a better plan and a more believable story.

The porcupine tale "was preposterous and almost completely unbelievable by virtue of its utter stupidity," Lock said. "It made no sense on several levels."

Lock agreed with Blessington that the extramarital affair between Scher and Patricia Dillon was "open and notorious" — and suggested that gave Dillon just as much motive for wanting to harm Scher as the other way around.

Dillon's death "was an accident occasioned by a struggle over the weapon," Lock said.