Idaho dad sentenced to life for killing 18-year-old daughter

Delbert Cornish pleaded guilty to first-degree murder and other charges in the death of his daughter Onyx

Jan. 29, 2026 at 1:23pm

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Delbert Cornish, a 51-year-old Idaho man, was sentenced to life in prison without the possibility of parole for killing his 18-year-old daughter Onyx in August 2025. Cornish pleaded guilty to first-degree murder and other charges, admitting he shot and killed Onyx in their home while his 15-year-old son Lucian was present. Lucian was able to get the gun away from his father and call 911, but Onyx died at the scene. Cornish had intended to kill both of his children and then himself, but only succeeded in killing Onyx.

Why it matters

This tragic case highlights the devastating impact of domestic violence and the trauma experienced by family members, especially children, who witness such horrific acts. It also raises questions about mental health support and intervention for individuals struggling with depression, suicidal ideation, and violent impulses.

The details

According to court documents, on August 18, 2025, Delbert Cornish grabbed a handgun, went to his 18-year-old daughter Onyx's room, and shot her in the head. Onyx died at the scene as Cornish watched her gasp for breath for 20 minutes without calling for help. Cornish's 15-year-old son Lucian was home at the time and fought with his father over the gun before fleeing and calling 911. Cornish later admitted to killing Onyx and intending to kill Lucian as well before taking his own life.

  • On August 18, 2025, Delbert Cornish killed his 18-year-old daughter Onyx.
  • In the months following the incident, Cornish pleaded guilty to first-degree murder and other charges.

The players

Delbert Cornish

A 51-year-old Idaho man who pleaded guilty to first-degree murder and other charges for killing his 18-year-old daughter Onyx.

Onyx Cornish

The 18-year-old daughter of Delbert Cornish who was killed by her father in their home in August 2025.

Lucian Cornish

The 15-year-old son of Delbert Cornish who was present when his father killed his sister Onyx and fought with his father over the gun before fleeing and calling 911.

Crystal Thompson

The mother of Onyx and Lucian Cornish, who described her daughter as a "light in this world" and said her ex-husband's actions have "permanently destroyed" her meaning of family.

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What they’re saying

“Onyx deserved to grow up.”

— Crystal Thompson, Mother of Onyx Cornish

“I want to start by saying that the day my sister died is the day I didn't feel a spark anymore.”

— Lucian Cornish, Son of Delbert Cornish

“I'm not gonna lie, I did it, dude.”

— Delbert Cornish

What’s next

Delbert Cornish will be transferred from the Canyon County jail to one of Idaho's state prisons to begin serving his life sentence with no possibility of parole.

The takeaway

This tragic case underscores the devastating impact of domestic violence and the urgent need for improved mental health support and intervention to prevent such horrific acts. The trauma experienced by Onyx's family, especially her young brother Lucian, is a sobering reminder of the far-reaching consequences of violence within the home.