Weston, who has long suffered bouts of paranoid schizophrenia, appeared to have trouble understanding the charge against him and that he could receive the death penalty if found guilty, defense attorneys said in court filings.
Kristina Swickard, social services coordinator for the Salt Lake Legal Defender Association, which represents Weston, met with him at the Salt Lake County Jail and found he appeared psychotic. He suffers from auditory hallucinations and tends to mistake the information "supplied to him by his voices" for reality, according to court findings.
Weston has a history of outbursts that predate his grandmother's murder.
In August, he was accused of punching one University of Utah Neuropsychiatric Institute employee in the face and another in the ears.
A month later, he swallowed a rock and poured scalding water down his throat. He was admitted as a "psych patient" to LDS Hospital, where he jumped through a window into a nurses station and injured two employees who tried to restrain him, according to police.
In late September, Weston was arrested again. He was charged with misdemeanor assault after allegedly hitting his father, Walter Weston, and biting a woman who was in his Millcreek area home.
After his mother bailed him out, Weston went to stay with his grandmother. She was slain the next day, police said.
Judge Denise Lindberg ordered a mental health review for March 25, after Weston has spent 90 days at the state hospital.