Prosecutors in the case of Christopher Pittman, 15, were expected to call reply witnesses later in the day, and the murder case was likely to go to the jury Friday.
Pittman was 12 when he shot Joe Pittman, 66, and Joy Pittman, 62, to death with a pump-action shotgun as they slept in their rural Chester County home in November 2001.
Prosecutors say was angry at his grandparents for disciplining him. For its final witness Thursday, the defense presented testimony from a psychiatrist and former Food and Drug Administration official that the antidepressant kept Pittman from knowing right from wrong.
"The whole sequence of actions was rash and frantic and done at a high level of anger - anger that was chemically induced," said Richard Kapit, who at the FDA once handled applications and safety reviews of antidepressants like Zoloft.
Pittman suffered from a substance-induced mood disorder with psychotic features, Kapit testified.
The defendant was "very rash, very excited and very angry," Kapit said, adding his actions were "very much a part of manic behavior."
Pittman, who is being tried as an adult, faces 30 years to life in prison if convicted.
A month before the slayings, Christopher was hospitalized in Florida, where his father lives, when he threatened to kill himself. The boy was prescribed the anti-depressant Paxil. Shortly after he moved in with his grandparents in early November 20001, another doctor put him on Zoloft.
Prosecutors say Pittman killed the couple, then burned their house and drove about 20 miles in their car before getting bogged down on a road. He initially told police a black man killed his grandparents and kidnapped him.
In a statement to police, Pittman said his grandparents deserved to die because they paddled him. Killing someone simply because they punished you, Kapit said, is just another sign Pittman was manic.
The trial was moved to Charleston County because of pretrial publicity in Chester.
By Bruce Smith