Court cites “act of pure evil” in slaying of 18-year-old Hope “Onyx” Cornish and attack on her younger brother.
CALDWELL, Idaho — Third District Judge Gabriel McCarthy on Jan. 28 ordered Delbert Cornish to serve life in prison without the possibility of parole for killing his 18-year-old daughter, Hope “Onyx” Cornish, and attempting to kill her 15-year-old brother at the family’s Caldwell home on Aug. 18, 2025.
The ruling followed a sentencing hearing where prosecutors pressed for the maximum term and the defense asked for a 25-year fixed sentence. McCarthy agreed with the state, denouncing the crime as “an act of pure evil.” The Canyon County Prosecutor’s Office credited the Caldwell Police Department, the Canyon County Sheriff’s Office and the Coroner’s Office for their work. The decision closes the punishment phase in a case that has gripped the Treasure Valley for months and left a surviving teen and relatives navigating their loss.
In court, Chief Deputy Prosecutor Ingrid Batey described the attack as “senseless, cruel, cowardly, and callous,” arguing that only a life-without-parole term would match the harm done. Deputy Prosecutor Stephanie Morse assisted the state. The judge also referenced a related animal-cruelty count tied to the shooting of the victim’s pet cat, one of several aggravating details aired in the courtroom. Defense attorney Erik Ellis urged the court to weigh the defendant’s depression and suicidal ideation, asking for a 25-year fixed term, but the court found the facts and risk factors outweighed those claims.
The case file places the killing on Aug. 18, 2025, inside a Caldwell residence. Onyx Cornish died of a gunshot wound; her younger brother survived an attempt on his life. The prosecutor named the brother as 15 at the time of the attack. Prosecutor Chris Boyd later thanked investigators and said the sentence ensures the defendant “wakes up behind bars every day for the rest of his life.” The courtroom was full during the hearing, and several people spoke in soft tones afterward about the teen’s creativity and kindness. The court did not announce any special conditions beyond the statutory terms and fees set in the judgment. Whether any post-conviction motions will follow is unknown.
Records show the matter progressed quickly from charging to sentencing over five months. Idaho law allows for life without parole in first-degree murder cases under qualifying circumstances; prosecutors did not seek the death penalty here. The judge’s order sets life without parole on the murder count and consecutive time for attempted murder and animal cruelty. The clerk entered the judgment the same day. The Department of Correction took custody after deputies escorted Cornish from the courtroom, beginning an automatic classification and intake process common to life sentences.
Outside the courthouse, family members asked people to focus on the teenager’s life. “We want people to remember her for her life, not for her death,” her mother said. A neighbor who lives a few doors away said the case “shook the block” and that seeing the court phase end “helps the whole street breathe.” By evening, a few flowers and handwritten notes rested near the courthouse steps.
As of Friday, the sentence remains in effect, with no parole eligibility under Idaho law. Any appellate notice would follow normal deadlines. No further hearings were on the docket as of the last check.
Author note: Last updated February 1, 2026.