Weston, who had been living with the woman, was standing next to her in bloody clothes and holding a bloody knife, according to the charging documents filed in 3rd District Court.
Weston later told police he slit the woman's "belly, her jugular and her heart," the charges state. Doctors said the woman had been stabbed nine times in the heart, and some of her organs had been removed.
Prosecutors cited the "especially heinous" nature of the slaying in charging Weston with the capital offense.
Weston's family has said the man suffers from mental illness, calling his life a "tragedy in slow motion."
"Joyce was a martyr by any definition," Weston's uncle, Charles Kulander, previously told The Salt Lake Tribune. "If anybody was closer and loved Zach more than her, I don't know how. She was always there for him. The rest of us had just had it up to here [with Weston], but Honey always kept her door open."
Weston had moved in and out of relatives' homes since he was a teenager. "Zach was always a sweetheart, but he started receding into his mental illness to where he was almost catatonic," Kulander said. "He was withdrawn, lethargic."
But eventually, the man started becoming more aggressive.
"It was just too much," Weston's aunt, Jil Kulander, said. "We said, 'No more.' Zach wasn't allowed to stay [with Dexter]."
The man was diagnosed with schizophrenia and told others he was hearing voices, family members said.
Weston was charged in October 2010 with misdemeanors after he allegedly shoved his mother and punched two police officers as they tried to remove him from a 7-Eleven where he had been loitering.
In August, Weston allegedly punched one University of Utah Neuropsychiatric Institute employee in the face, and another in the ears.
In September he swallowed a rock and poured scalding water down his throat, after which he was admitted as a "psych patient" at LDS hospital. There, he jumped through a window into a nurses station and injured two employees trying to restrain him, according to a police report.
Later in the month, Weston was arrested again for allegedly hitting his father and biting a woman who was in his Millcreek area home.