Background: Delbert Cornish appears in Canyon County, Idaho, court on Wednesday, Sept. 17, 2025 (KIVI/YouTube). Inset: Hope “Onyx” Cornish and her cat (GoFundMe).
An Idaho father who allegedly killed his adopted daughter and had plans to kill his other children appeared in court this week, as the case against him appeared to expand.
Delbert Cornish, 51, is accused of first-degree murder in the killing of 18-year-old Hope “Onyx” Cornish. On Wednesday, he appeared in Canyon County court, looking stoic as he had one charge added and another upgraded against him.
The defendant is now charged with assault with intent to commit murder — instead of aggravated assault — and he also faces a misdemeanor animal cruelty charge for allegedly killing his daughter”s cat, Boise ABC affiliate KIVI reported. Delbert Cornish is also charged with possession of a deadly weapon.
Law&Crime previously reported on the “horrific act of family violence.” On Aug. 18, at a home on the 1600 block of Blaine Street in Caldwell, a suburb of Boise, Delbert Cornish took several shots of alcohol around 9 a.m., according to court documents obtained by local outlets including KIVI. After doing so, he entered his daughter’s bedroom and allegedly shot her in the side of the head.
The defendant then kissed her on the cheek and shot her cat “for the same reasons, because no one would take care of it after they were all dead,” the probable cause affidavit says. Delbert Cornish allegedly proceeded to cover Hope Cornish’s body with a large pride flag “as a way to honor her, because it was something that meant a lot to her.”
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He had been planning to kill his daughter, two sons, and himself for nearly a year, ever since he bought a black 9 mm Hi-Point handgun in September or October 2024, police have said. And on that August day, as the hours passed after the shooting, Delbert Cornish allegedly waited for his youngest son — who is reported to have been either 14 or 15 years old at the time — to come home from school.
When the teen arrived, the defendant reportedly asked him how school was before telling him “he had killed his sister.” Delbert Cornish also tried to convince the boy to “check on his sister,” but the son refused, the affidavit states.
“Delbert then went to the couch [and] grabbed a gun,” at which point his son said he wanted to leave, the court document adds. But the defendant did not intend to let that happen, authorities maintain. He is said to have “shot at the ground,” blocked the doorway, and pointed the gun at his son’s chest.
They fought over the gun, according to the child, with the son eventually winning control over the firearm. The teen then ran away from the house to call 911.
When law enforcement officials arrived at the home after 4 p.m., they found the magazine, the handgun, and a “large note written on the wall” that appeared to be a possible murder-suicide note. The note allegedly included statements such as “I’m a man of my word my kids died by my hand. I’m tired of them being made fun of,” and “I’m going! My kids are going with me.”
Police also found Hope Cornish and her cat dead. Delbert Cornish was arrested about 70 miles southeast heading to his ex-girlfriend’s home, local NBC affiliate KTVB reported.
“I’m not gonna lie, I did it dude,” Delbert Cornish allegedly said unprompted as an officer was preparing to test him for gunshot residue (GSR). He then asked questions about the crime scene and whether investigators had found his daughter’s body, according to the affidavit. When the officer replied in the affirmative, the suspect reportedly “began to cry.”
“Delbert asked further questions about the gun and told me he should’ve shot himself, as well as said he didn’t want to shoot his son,” the detective in the affidavit added. “I later collected the GSR sample from each of his hands. While collecting the sample from his left hand, Delbert informed me that he shot the gun with his right hand.”
The affidavit goes on, in detail, about an alleged motive.
“Delbert admitted to me that he is an alcoholic who struggles with depression,” it states. “Delbert said he is tired of living and is tired of being ‘belittled’ by his brother, whom he and his children live with. Delbert advised he is tired of his children being bullied by [his brother] and referenced named he calls them[.]”
Delbert Cornish allegedly then outlined that he wanted to kill his three children – including a 24- or 25-year-old son – but the plan was “thwarted” when this older son violated his parole a few months ago and was sent back to prison.
“Delbert explained he wanted to kill his children along with himself because he believed nobody, especially [his brother], would take care of them once he was dead, and that they would not be able to carry on in life without him,” the detective wrote in the court document. “Because of this, he believed they would all be better off dead.”
A GoFundMe posted for “Onyx” Cornish begins with a firm accusation: “Onyx was killed [by] her father.”
“The road ahead is going to be a lengthy one, and L.C. [the teenage son] is carrying so much weight on his heart and shoulders because he lost both his sister and his father that terrible day,” it goes on, adding that Hope Cornish was “especially protective of her youngest brother, who has autism.”
The person behind the fundraiser appears to be the mother of both Hope Cornish and her teen brother.
“Our family has faced unimaginable trauma. My oldest son, L.C., was also tragically affected by his father. The killer intended to take both of their lives before his own, but L.C. bravely fought back, managed to escape, and called 911,” the fundraiser adds.
Delbert Cornish is scheduled for an arraignment hearing on Oct. 14.
