Castillo is claiming the insanity defense in the murder and assault charges against him.
Testimony seemed to suggest Castillo repeatedly sought help from his family, but to no avail.
For the second day Castillo's mother, Vicky, testified on behalf of her son and said he wanted his father to forgive him.
The multiple gunshots to her husband's head, according to Vicky Castillo, were her son's attempt to end his father's pain.
"He got very nervous, because he didn't want him to suffer and then because he got very nervous and started shooting with the goal of not making him suffer," she said.
Castillo's mother denied she was aware of her son's violent plan and that she was more afraid he would hurt himself rather than anyone else.
According to a defense witness, his behavior that day in August 2006 might have had a lot to do with a family history of mental illness.
A mental health expert outlined for the jury multiple relatives with depression, anxiety and alcoholism.
His volatile home life, social worker Deborah Grey said, only complicated matters.
"He went from being happy boy on the shy side but interactive, to a very isolated child who wouldn't play with the others and part of that was fear of being watched and part of it was fear of being teased," Grey said.
As a young adult depression set in along with confusion over his sexuality, but it was his father's ongoing abuse according to the defense that may have pushed him over the edge.
"He loved his father and he very much wanted his approval and he also was very much afraid of him," Grey said.
The defense will continue Thursday with more expert testimony.