‘Onyx deserved to grow up’: Idaho dad sentenced for killing 18-year-old daughter

BOISE (Idaho Statesman) — Lucian Cornish lost his spark on Aug. 18, 2025.

When the 15-year-old got home from school that afternoon, he immediately knew something was very wrong, Canyon County Chief Deputy Prosecutor Ingrid Batey said in court Wednesday. His father, Delbert Cornish, asked him about his day and then told him that he’d killed his older sister, 18-year-old Hope “Onyx” Cornish.

As Lucian would later tell law enforcement, he was in “absolute shock,” Batey said.

He asked his father what he was talking about, which is when Cornish pulled out a handgun, telling his son to go into his sister’s room and see that she was dead, Batey said. Lucian refused, begging his father to let him leave. The pair fought over the gun, and Lucian was able to get it away from his father before dropping the gun, running from the home and calling 911, according to Batey.

Caldwell police officers arrived to the 1600 block of Blaine Street, blocked off the house, and announced themselves to try to get Cornish outside. Later, officers said they learned that Cornish was no longer there, and in the house they found Onyx shot to death inside her bedroom.

Cornish was quickly apprehended and admitted to killing his daughter within hours of her death, telling the police corporal who arrested him, “I’m not gonna lie, I did it, dude,” according to the probable cause affidavit.

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Within months of his arrest, Cornish pleaded guilty to first-degree murder in Onyx’s death, along with several other charges, including felony assault with intent to murder and misdemeanor cruelty to an animal, court records showed. He was also convicted of an enhancement for the use of a deadly weapon in the commission of a felony, which can add 15 years to someone’s prison sentence.

For Crystal Thompson, Onyx and Lucian’s mother, her ex-husband’s actions have “permanently destroyed” her meaning of family, she said in court Wednesday.

Wearing a black T-shirt adorned with the number three — Onyx’s favorite number — Thompson described her only daughter as one of the most “vicious and forthright” people she’d ever known. Someone who was bold, fearless, full of life and loved her family, especially her brothers, deeply, she said.

“Onyx deserved to grow up,” her mother said.

On Wednesday, Cornish, 51, was sentenced to life in prison without the possibility of parole by 3rd District Judge Gabriel McCarthy, who presides over cases in Canyon County. There are some criminal cases where punishment isn’t the main consideration when deciding someone’s prison sentence, but that wasn’t the case when deciding Cornish’s punishment, he said.

Despite arguments from Cornish’s public defender, Erik Ellis, for his client to be eligible for parole after serving 25 years, McCarthy said he wasn’t persuaded. Cornish was an alcoholic and had dealt with depression and suicidal ideation for decades, Ellis said. While there are “zero excuses” for what his client did, he added, he wanted to explain what made Cornish “tick.”

But Onyx’s killing was “an act of pure evil,” McCarthy said.

While Ellis articulated every possible factor that could be in Cornish’s favor, the judge said it didn’t add up to much compared to the harm of his criminal actions. Cornish is currently incarcerated at the jail in Canyon County, but will be transferred to one of the state’s prisons to begin serving his lifetime sentence.

“He will remain there until he passes this Earth,” McCarthy said. “If there is a God, perhaps God will have mercy on his soul — the state of Idaho does not.”

Affidavit: Cornish planned to kill both his teenage children

Cornish said he could never muster enough words to convey the amount of “regret, anguish and remorse,” he’s caused his family. Standing with his hands cuffed to a belly chain around his waist, he apologized for killing his daughter and planning to kill his son, and said that it’s a debt he’ll never be able to pay back.

Cornish intended to shoot himself and decided he’d also kill his children, according to the affidavit. He told law enforcement that his plan for the day was for both of his teenage children, Onyx and Lucian, to be dead and for himself to be dead, too.

At around 9 a.m. on Aug. 18, Cornish grabbed his black 9 mm Hi-Point handgun, walked down the hallway to his daughter’s room and stood in front of her door, according to the affidavit. While he was standing outside her bedroom, Onyx opened the door and “startled him,” according to the affidavit.

He then shot her through the head and watched her gasp for breath for 20 minutes — never calling for medical help, Batey said in court Wednesday. “Onyx died there, surrounded by her clothes, some personal items, and a stuffed teddy bear,” the county prosecutor said.

On a daily basis, Lucian has wondered whether he could’ve done something to save his sister, he wrote in a statement, which was read on his behalf Wednesday by Canyon County Deputy Prosecutor Stephanie Morse. Some days he can’t make it to school, he has visions of his father pulling the trigger, and deals with waves of anxiety and fear that hit him “like a truck,” he said.

“I want to start by saying that the day my sister died is the day I didn’t feel a spark anymore,” Lucian Cornish wrote. His father, dressed in an orange jumpsuit, rocked back and forth in his chair with several tears streaming down his face, and listened as his son’s statement described the impact that losing his sister had on his life.

The thing Lucian can’t get out of his head is why his father would kill his own daughter for his own selfish gain, he wrote. He called it the most “cowardly decision” he’d ever seen.

“I still have a life, a future and goals. Why would the person who was supposed to show me what a man is, do that to me?” Lucian wrote.

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