An Idaho man has been sentenced to life in prison for the murder of his daughter and the attempted murder of his son, crimes described in court as “pure evil.” Delbert Cornish, 51, received his sentence on Wednesday during an emotionally charged court session.

Court records from Canyon County, reviewed by Law&Crime, confirm that Cornish pleaded guilty in November to multiple charges, including first-degree murder of his 18-year-old daughter, Hope “Onyx” Cornish. He also admitted to assault with intent to commit murder, cruelty to animals, and using a deadly weapon in the commission of a felony.

In exchange for his guilty plea, prosecutors agreed to remove the death penalty from consideration.

Following the sentencing, Crystal Thompson, the mother of the victim, expressed relief to Boise’s NBC affiliate KTVB. “Now my family and I can begin to start putting this behind us,” she said.

During the hearing, Thompson shared her ongoing struggles with Judge Gabriel McCarthy. “I am struggling to get through every day,” she admitted. “Sometimes the reality… it constricts my chest and I can’t breathe. I can’t breathe. It’s hard to get through my shifts at work, hard to sleep, hard to wake up.”

Thompson, who had been in a long-term relationship with Cornish and built a family with him, expressed her shock and betrayal in court, stating she had trusted him and never imagined he could harm their children.

“All I can see in those memories are that I chose wrong, and I can’t go back to save her,” she added.

On Aug. 18, 2025, a “horrific act of family violence” occurred, as authorities would later recount. At a residence 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 KTVB. He then entered his daughter’s bedroom and shot her in the side of her head.

Delbert Cornish 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,” a probable cause affidavit said. The defendant then covered 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.”

Background: The 1600 block of Blaine Street in Caldwell, Idaho (Google Maps). Inset: Delbert Cornish (Canyon County Sheriff’s Office).

According to authorities, the man 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. And on that August 2025 day, as the hours passed after the shooting, Delbert Cornish waited for his youngest son, 15-year-old Lucian Cornish, to come home from school.

When the teen arrived, Delbert Cornish asked him how school was before telling him “he had killed his sister.” He then tried to convince the boy to “check on his sister,” but the son refused, court documents stated.

“Delbert then went to the couch [and] grabbed a gun,” at which point his son said he wanted to leave, the court document added. But the defendant did not intend to let that happen, authorities maintained. He “shot at the ground,” blocked the doorway, and pointed the gun at his son’s chest.

They fought over the gun, according to Lucian Cornish, with the son eventually winning control over it and running away 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. It 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.”

Responding police officers found Hope Cornish and her cat dead. They then caught up with Delbert Cornish about 70 miles away, and when they spoke with him, authorities say he confessed to killing his daughter and “began to cry.”

“I’m not gonna lie, I did it dude,” the defendant said, unprompted.

Authorities believe Delbert Cornish killed his daughter and tried to kill his sons because he “believed nobody … would take care of them once he was dead, and that they would not be able to carry on in life without him.”

Lucian Cornish was reportedly not in court, though a deputy prosecutor read his impact statement out loud.

“Sometimes I have visions of my own father actually pulling the trigger,” he wrote. “Something I can’t get out of my head is someone would kill someone they love for their own selfish gain.”

The convicted man, for his part, apologized in court on Wednesday.

“I could never muster enough words to convey the amount of regret, anguish and remorse I have for what I’ve done to each and every one, including those that never had a chance to meet her,” Delbert Cornish said, per video shot by the local outlet. “That’s a debt I could never pay back in this lifetime or the next.”

The judge, though, did not strike a forgiving tone, describing the defendant’s actions as “pure evil.”

“If there is a God, perhaps God will have mercy on his soul,” the judge said. “The state of Idaho does not.”