A Scottsdale man accused of beating his 67-year-old roommate to death has been found incompetent to stand trial after a judge concluded there was no clear evidence he could be restored to competency within 15 months. Matthew Dieringer, 36, is accused of killing Frank Quaranta, a man he met at a heat relief center and later moved in with for several months before Quaranta turned up dead in his own Scottsdale home.
The finding caps months of competing mental-health opinions in the case, according to FOX 10 Phoenix, which reported that two mental health experts had previously said Dieringer was competent to stand trial, while a third diagnosed him with delusional disorder, compulsive personality disorder, a narcissistic personality type and a paranoid personality style. The outlet reported that Dieringer has been found competent in two previous Arizona cases, but that a mental health expert described him as disruptive and erratic, and that he was uncooperative and irrational — and experienced delusional thoughts — when not medicated. At one hearing, Dieringer reportedly told the court, “Here comes the Rule 11. It's all scripted!” a reference to the Arizona Rule 11 proceedings used to evaluate a defendant's competency.
The judge ultimately found there was no clear evidence Dieringer would be restored to competency within 15 months, FOX 10 Phoenix reported, and he has been placed in a restoration competency program within Maricopa County Correctional Health Services. His next court hearing is scheduled for November 12, 2026.
A Deadly End to a Months-Long Living Arrangement
Quaranta, who worked at the heat relief center where the two men met, was found dead in his Scottsdale home, according to azfamily.com, which reported that he was discovered during a police welfare check. The medical examiner found that Quaranta suffered multiple skull fractures, a broken jaw, a neck fracture and broken ribs, the station reported. Dieringer had been living with Quaranta for several months at his home after the two connected through the center, according to abc15.com, which reported that the state is pursuing a first-degree murder charge against him.
Dieringer was arrested at a park in Phoenix on Sept. 6 after police received a tip from a man who recognized him while he was walking his dog, abc15.com reported. Scottsdale police described him as charismatic and manipulative, the station's report noted.
A Trail of Prior Violence Involving Animals
Court records reviewed by FOX 10 Phoenix describe an incident in which Dieringer slammed a woman's dog to the concrete, then tried punching a man roughly ten minutes later and stole the man's glasses. By April 2025, he had pleaded guilty to theft and was placed on supervised probation requiring mental-health screening, per the same outlet's reporting.
That wasn't his only run-in involving animal violence. Dieringer was convicted in Colorado in 2023 of felony animal cruelty and sentenced to 18 months in prison after mutilating and killing his roommate's two dogs, according to denver7.com.
Delusions of Celebrity and a Fired Attorney
FOX 10 Phoenix reported that Dieringer believed he was the target of a conspiracy tied to what he described as his own celebrity status, and said he had written a letter to the FBI and was awaiting a response. He also grew frustrated with his attorney's refusal to validate what he considered his social media influence as relevant to his defense, the station reported.
That friction reportedly boiled over in court, where Dieringer requested that his attorney be fired, saying he did not trust the lawyer to represent him. His defense attorney told the court that Dieringer would not talk to him, according to the outlet's reporting, and the judge advised Dieringer to file a motion explaining why he wanted to change counsel.
How Arizona Handles Incompetent Defendants
The competency process playing out in Dieringer's case follows Arizona Rule 11, which governs evaluations and restoration proceedings for people charged with crimes, according to a Journal of the American Academy of Psychiatry and the Law analysis. Under Arizona law, a court that initially finds a defendant incompetent must order restoration treatment unless there is clear and convincing evidence the defendant cannot be restored within 15 months, according to the Arizona Legislature's statute. The same body of law specifies that the mere presence of a mental illness, defect or disability is not by itself grounds for an incompetence finding.
People found incompetent but considered restorable can be committed to jail-based or community-based restoration programs, the journal noted, a system Arizona began building in 2003 partly because of long waits for hospital beds and the lower cost of jail-based programs compared to hospitalization. By spring 2018, the state had five county jail-based restoration programs, and the Arizona State Hospital averaged only about 10 competency-restoration admissions per year between 2013 and 2017, according to the same analysis.
Dieringer's case will return to court on November 12, with his placement in the Maricopa County restoration program now the central fact of his legal status heading into that hearing.
