Editor’s note: This story contains graphic details of a homicide.
Shortly after Mountain Home police arrested him, the 51-year-old father accused of killing his teenage daughter admitted to the crime, according to a probable cause affidavit obtained by the Idaho Statesman.
Delbert Cornish was arrested Monday in Mountain Home after he fled from his brother’s home in downtown Caldwell, where he lived with his daughter and one of his sons, after law enforcement alleged he shot and killed his 18-year-old daughter, Onyx Cornish.
He was charged with first-degree murder, as well as aggravated assault and an enhancement for the use of the deadly weapon in commission of a felony, according to a criminal complaint filed by the Canyon County Prosecutor’s Office.
He’s now being held at the Canyon County Jail without bond, online records showed. If Cornish is convicted of first-degree murder, he could face up to life in prison or the death penalty, if prosecutors choose to seek it. His next hearing is scheduled for 8:30 a.m. Sept. 3 at the Canyon County Courthouse.
‘She was just a kid’: Mother wonders why Onyx was killed
When Cornish’s 15-year-old son got home from school Monday, Cornish asked the boy how his day was and then told the teen that he’d killed his sister, Caldwell Police Detective Taylor O’Neil wrote in the affidavit. Cornish tried to get his son to go into his sister’s room to see that she was dead, but he refused, the affidavit said.
That’s when Cornish grabbed his handgun from the couch, shot the gun at the floor and blocked the teen from leaving the house, O’Neil wrote. Cornish also pointed the firearm at his son, according to the affidavit. They fought over the gun, and his son was able to get it away from his father before dropping the gun in the doorway, running from the home and calling 911, according to the affidavit.
Officers from the Caldwell Police Department arrived at the scene in the 1600 block of Blaine Street, blocked off the house, and announced themselves to try to get Cornish out of the house, the affidavit said. Later, officers said they learned that Cornish was no longer there and had taken his brother’s 2020 Orange Dodge Challenger to see his ex-girlfriend in Mountain Home.
Cornish was arrested just before 7 p.m. and read his Miranda rights, informing him that he had the right to remain silent, and anything he said could be used against him. While police drove him to the station, Cornish began talking about the shooting, according to the affidavit.
He “admitted to murdering his daughter” when a Mountain Home police corporal sat him down in the station’s interview room, the affidavit said. He again admitted to committing the crime when Detective O’Neil entered the room.
“I’m not gonna lie, I did it, dude,” Cornish said, without being questioned, while O’Neil swabbed his hands for gunshot residue, according to the affidavit.
The Caldwell Police Department declined to answer several follow-up questions about the case sent by the Statesman.
“There is still a need not to release the entire case and details, including that the suspect has the right to a fair and unbiased jury,” spokesperson Char Jackson wrote in an email. “When the case is adjudicated, we can possibly release this information.”
Cornish began crying when he asked whether they’d found his daughter, and said he should’ve killed himself, the court filing said. Officers found Onyx lying on the floor of her bedroom with a gunshot wound to her head, according to the affidavit.
Onyx was known to her family and friends as a strong-headed and outspoken young woman who loved music — particularly heavy metal. Her mother, Crystal Thompson, who passed her musical tastes onto her daughter, said some of Onyx’s favorite bands were Avenged Sevenfold and Bullet For My Valentine.
Onyx had dealt with substance abuse issues in the past, but her mother said she was sober for over a year and started dating someone new who “brought out the best in her.” Her mother said she couldn’t understand why this happened.
“I keep thinking, ‘Was she scared?’” Thompson told the Statesman. “’Was there a big fight? Was he mean to her?’ That’s what’s eating at me: I don’t know how she felt.”
While Cornish never had a formal mental health diagnosis that Thompson was aware of, he’s dealt with depression for years, the woman told the Statesman. But as parents, she said, it’s their responsibility to deal with their own trauma and issues.
“We are not to put that on our children. Like, how dare he?” Thompson said. “I just don’t know what to make of it, and I can’t understand why he wouldn’t do something else: ask for help, go for a drive, beat up the couch. I don’t care.”
“I just wonder how long he knew he was going to do that,“ she added. “I don’t know, why? Why? She was just a kid.”
Cornish admits to killing daughter, court record says
In Cornish’s interview with O’Neil, Cornish detailed his alcoholism and his struggles with depression and said he was tired of living, according to the affidavit. He said bought a black 9 mm Hi-Point handgun from Sportsman Warehouse in Nampa last year, and that he intended to shoot himself and decided he’d also kill his children, the court document said.
When O’Neil asked Cornish what his plan for the day was, Cornish responded, “Both of my kids would be dead, and I’d be dead,” according to the affidavit. Cornish told O’Neil that once his son left for school Monday, he took a couple of shots of bourbon and sat on the couch for around an hour contemplating his decision, according to O’Neil.
At roughly 9 a.m., he grabbed the handgun, walked down the hallway to his daughter’s room and stood in front of her door, the affidavit said. Onyx worked nights at the Wendy’s off of 10th Avenue in Caldwell, and Cornish told O’Neil that he thought about killing her in her sleep because it “would be easier, right?” the affidavit said.
While he was standing outside her bedroom, Onyx opened the door and “startled him,” according to the affidavit. He then shot her. “For the next 20 minutes, I watched her gasp for breath,” Cornish told O’Neil as he cried. “I’m a piece of s**t, man. I’m not going to lie, I did it.”
He then told Onyx he was sorry, kissed her on the cheek and covered her in a large Pride flag as “a way to honor her,” according to the affidavit. He also shot and killed her cat.
Cornish said he went into a “fit of rage” afterward, according to the affidavit. He punched several holes in the wall and swung a hatchet into Onyx’s bedroom door, O’Neil wrote in the affidavit. Cornish began to drink more to “forget about what he had just done,” and drank roughly a third of a half-gallon bottle of bourbon, the affidavit said.
He also left several suicide notes throughout the house, including a handwritten note he wrote weeks ago and hid in the living room, according to the affidavit.
“I am Delbert Cornish. I have done the abomination you find (here),” the signed note read. “My daughter … and my son … have gone with me by my own accord. No one will help them if I’m gone. I hereby check out of this life. Depression is a b***h.”
When his son got home from school, Cornish told him he’d shot Onyx, and tried to prevent him from leaving the house, the affidavit said. Cornish denied ever pointing the gun at him.
Cornish agreed to write an apology letter to his son during the interview, the affidavit said. He apologized for his actions, told him he loved him and was proud of him for standing up to him, and told his son to “never be like him,” the affidavit said.
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