He has pleaded not guilty by reason of insanity. But the trial took a dramatic turn in the form of a confession Friday via a video diary.
"I did it," Castillo said in the diary. "I killed my father. I sacrificed him. Let the Lord know. He's with Tony now. It's time to get this done. Once again, I'm sorry for what I did. I am sorry, but look at me. I'm not even crying now. I just killed him and I feel fine. I feel fine. I feel fine."
The video, which was produced by Castillo, played for jurors in the courtroom, he talks of his obsession with the Columbine High School massacre. He had a copy of a documentary about the event.
"Whenever I watch it, it's fun. Does that mean I'm sadistic? I don't know. Does it mean I'm a psychopath? I don't know. Does it mean I'm insane? Probably," he tells the camera.
Castillo says he made the recording for detectives to find after his death.
Dressed in a black trench coat, he shows off his weaponry and demonstrates how to build a pipe bomb. He also says that he's not a psychopath - just a depressed teenager.
But later, he appears to contradict himself.
"Am I crazy? I think I am. I don't want to go to a mental hospital or live my life in an asylum. And I don't want to go to jail: prison, being raped, murdered, extorted," he says.
If caught, Castillo says he'd choose the death penalty.
"If you sentence me to death, I will petition for it to come as soon as possible," he says.
Castillo, yelling into the camera, claims he was in extreme emotional pain brought on by his abusive father.
"Am I a monster? I don't know. I was turned into one after years of abuse," he says.
At times, he's in he's in tears - other times angry.
"It's over. I have been like this for years," he says. "I just want to end it. Can't you understand that?"