CANYON COUNTY, Idaho (Scripps News Boise) — A lawyer representing Delbert Cornish, the Caldwell father charged with murdering his 18-year-old daughter, Onyx Cornish, in August, told a Canyon County judge on Tuesday that his client plans to plead guilty.
Delbert Cornish appears in court for a hearing on Oct. 14, 2025. (Victoria Rodriguez/Scripps News Boise)
Cornish is charged with first-degree murder, assault with intent to murder, use of a deadly weapon in the commission of a felony, and animal cruelty. Police say that after Cornish killed Onyx, he shot and killed her cat.
Canyon County neighborhood reporter Victoria Rodriguez was in court for Cornish’s arraignment on Tuesday and spoke exclusively with the victim’s mother, who considers the guilty plea a major relief for her surviving son.
Crystal Thompson, Onyx’s mother, described seeing Cornish for the first time since her 18-year-old daughter was shot dead inside a Caldwell Home. “He just came out and my blood went cold,” she said before adding, “I started shaking, you know, and I just felt— I felt despair.”
MORE | ‘Fight or die’ Teen recounts day Delbert Cornish allegedly killed daughter
As previously reported, court records obtained shortly after Delbert Cornish’s arrest indicate that the accused told police he planned to kill his children and then himself.
“He made eye contact with me, and then he looked away, and he never made eye contact again,” said Thompson. “He’s going to have to face it, you know, and it’s not fair.”
As a judge schedules a formal change of plea hearing for next month, Thompson says the arraignment brought mixed emotions — a small sense of relief, but also painful reminders of everything her family’s lost.
She says her biggest concern has always been protecting Onyx’s brother, Lucian, who police say ultimately disarmed his dad that day and called 911.
“It’ll save Lucian from having to go through, you know, getting up on the stand, that’s what I’ve been really worried about,” explained Thompson. “I just want my daughter to have justice. That’s why I’m here. I’m representing her because she can’t represent herself.”
Cornish is expected back in court on Nov. 12 to formally change his plea.
Thompson tells Idaho News 6 that she and her son, Lucian, will be there, as they continue to seek justice for Onyx and healing for their family.
This story was originally written by Victoria Rodriguez for Scripps News Boise, an E.W. Scripps Company.
