Zachary Cole Weston, 21, was charged with aggravated murder Friday in the Oct. 3 slaying of Joyce "Honey" Dexter, The Salt Lake Tribune reported.
Court documents revealed Weston, who had been living with his grandmother, was found standing next to her in bloody clothes and holding a knife.
Doctors said some of the woman's organs had been removed.
Weston's family said he suffers from mental illness and hears voices.
"Zach was always a sweetheart, but he started receding into his mental illness to where he was almost catatonic," an uncle, Charles Kulander, said. "He was withdrawn, lethargic."
Weston's lethargy later turned to aggression, the newspaper said.
In October 2010, he was charged with shoving his mother and punching two police officers who tried to remove him from a convenience store where he had been loitering.
In August, he allegedly punched two University of Utah Neuropsychiatric Institute employees.
Last month, the newspaper said, he swallowed a rock and poured scalding water down his throat, which landed him in a hospital, where he jumped through a window into a nurses station and injured two employees trying to restrain him, a police report stated.
Also last month, Weston was arrested for allegedly hitting his father and biting a woman.
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'
Benedict Cumberbatch's dramatic reading of R. Kelly lyrics is just what you need