Castillo has pleaded guilty by reason of insanity.
The witness also testified Monday that she didn't believe Castillo was psychotic, turning Castillo's trial into a battle between mental health experts.
The defense claimed Castillo suffered from depression and possible schizophrenia.
Some testimony claimed on the day he killed his father and then shot into a crowded lunch room at his former high school that he appeared psychotic.
But prosecution witness, UNC Psychiatrist Dr. Karen Graham disagreed.
"It was my best judgment that he did not have psychotic symptoms," Graham said.
After treating Castillo shortly after his failed suicide attempt in April 2006, she said he talked about having murdering fantasies, but never let on he would act them out.
Before his release, his parents were advised to remove all guns.
"It gets back to dangerousness," Graham said. "We had told him that he should not own a gun."
However, that didn't happen. Whether it could' have prevented Castillo's shooting spree months later was unclear.
According to the prosecution's expert, Castillo's family never mentioned his continued use of guns or his trip to Littleton, Colorado -the site of the Columbine massacre.
She said had she known that, she would have wanted more information from him.
"I would've felt I could break confidentiality rule, because again of the dangerousness if my concerns were still really strong, I could've had him involuntarily committed," Graham said.
Soon the jury will decide whether he suffered from a psychotic snap or that he knew exactly what he was doing when he pulled the trigger.