The judge then sentenced Christopher Pittman, now 15, to 30 years in prison.
His attorneys had argued that his mind had been clouded from his use of the anti-depressant Zoloft.
Pittman had already admitted that he shot and killed his grandparents three years ago, reports CBS News Correspondent Jim Krasula. His lawyers claim he didn't know what he was doing since he was taking Zoloft.
Joe Pittman, 66, and Joy Pittman, 62, were shot to death with a pump-action shotgun as they slept in their rural home in November 2001.
Prosecutors say the boy was angry at his grandparents for disciplining him.
The trial has been billed as the first case involving a youngster who says an antidepressant caused him to kill, and it comes at a time of heightened scrutiny over the use of antidepressants among children.
Defense attorneys urged the jury to send a message to the nation by blaming Zoloft for the killings. They said the negative effects of Zoloft, which is made by Pfizer Inc., are more pronounced in youngsters, and the drug affected Pittman so he did not know right from wrong.
"We do not convict children for murder when they have been ambushed by chemicals that destroy their ability to reason," attorney Paul Waldner said.
But prosecutors called the Zoloft defense a smokescreen, saying the then-12-year-old Pittman knew exactly what he was doing three years ago when he shot his grandparents, torched their house, and then drove off in their car.
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."
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.