But after deliberating all day Friday, the jury of six men and six women came out just before 5 p.m. and told the judge that they didn't buy that argument.
Instead, they found Castillo guilty of first-degree murder and a host of other charges.
The three-week trial got national attention and was featured on Court TV because of Castillo's admitted fixation on the Columbine school shootings. Castillo told investigators that he idolized the two teens responsible for the 1999 event.
Castillo lowered his head as the first guilty verdict was read and appeared disappointed.
The defense never tried to prove that Castillo was not responsible for shooting his father and firing a rifle into the high school. Instead, they argued he was mentally ill. Castillo tried to commit suicide just months before the shootings, and lawyers showed hours of rambling and bizarre video journals taped by the teen that showed him with guns.
Prosecutors said Castillo knew right from wrong and that the murder of his father and the attack on the school were well planned.
After the verdict was read, Castillo rose and spoke to the court. He apologized and even thanked his dead father.
"I would like to apologize to the victims that I hurt," he said.
The judge immediately sentenced him to life in prison without the possibility of parole.