In Caldwell, Idaho, a devastating act of violence has left a community in shock and a family shattered. On a seemingly ordinary Monday, August 18, 2025, the life of 18-year-old Hope “Onyx” Cornish was tragically cut short in a home on the 1600 block of Blaine Street. The suspect, her adoptive father, 51-year-old Delbert Cornish, now sits in Canyon County Jail without bail, charged with first-degree murder, aggravated assault, and possession of a deadly weapon during a crime. Court documents reveal a chilling confession and a harrowing tale of a man consumed by despair, culminating in an unthinkable act.
Onyx Cornish was a vibrant young woman who had recently turned her life around. Known to her family and friends as Onyx, she was a fierce spirit with a love for heavy metal music, her favorite bands including Avenged Sevenfold and Bullet for My Valentine. She had overcome struggles with substance abuse, maintaining sobriety for over a year, and was thriving with a new job at Wendy’s on 10th Avenue in Caldwell and a budding romance. Described by her mother, Crystal Thompson, as smart, beautiful, and fiercely protective of her younger brothers, Onyx’s life was full of promise. Yet, her adoptive father’s inner turmoil would cast a dark shadow over her bright future.
According to court documents, Delbert Cornish’s descent into darkness began nearly a year earlier, when he purchased a 9mm Hi-Point handgun from a Nampa sports store, intending to end his life and those of his three children. Struggling with chronic alcoholism and depression, Cornish had recently lost his job, a fact he concealed from his brother, with whom he shared the Blaine Street home. On the morning of the tragedy, he cryptically told his brother, “today was the day,” without revealing the sinister plan brewing in his mind.
As the day unfolded, Cornish’s actions grew increasingly erratic. After consuming several shots of alcohol around 9 a.m., he stood outside Onyx’s bedroom door, wrestling with his intentions. When Onyx unexpectedly opened the door, Cornish fired a fatal shot to her head. For 20 agonizing minutes, he stood by as she struggled to breathe, later admitting to detectives that he felt remorse but did not act to save her. In a bizarre attempt to “honor” her, he draped a Pride flag over her body—a symbol he knew was important to her—and shot her beloved cat, leaving it dead beside her. In a fit of rage, he also vandalized parts of the home, leaving behind a trail of destruction.
The horror escalated when Cornish’s teenage son returned home from school. According to the Probable Cause Affidavit, Cornish confronted the boy, admitting he had killed Onyx. He fired a shot into the floor and pointed the gun at his son’s chest, attempting to prevent his escape. In a courageous act, the teen fought back, disarming his father and fleeing to call 911. Police arrived just after 4 p.m., finding a handgun without its magazine on the living room couch, the magazine discarded near the front door, and a murder-suicide note scrawled on a wall between the living room and kitchen. The note, filled with angry messages to his brother, revealed Cornish’s plan to end his life and those of his children, believing they “would not be able to carry on without him.”
Cornish fled the scene in his brother’s car, driving to his ex-girlfriend’s home in Mountain Home, over 60 miles away. Using Automated License Plate Reader technology, Caldwell police tracked him down, and Mountain Home Police arrested him without incident at 7:30 p.m. that evening. During questioning, Cornish’s confession was stark and unprompted. “I’m not gonna lie, I did it dude,” he told detectives, later breaking down in tears as he asked about the crime scene and expressed regret for not shooting himself. He admitted to firing the gun with his right hand and revealed his struggles with depression and feeling belittled by his brother, with whom he and his children lived.
The community of Caldwell is left grappling with the aftermath of this “horrific act of family violence,” as described by Caldwell Police Chief Rex Ingram. Onyx’s mother, Crystal Thompson, expressed her anguish to the Idaho Statesman, saying, “At one point in time, I loved this man, but I’m just so angry—my daughter’s dead.” She now faces the heart-wrenching task of breaking the news to her youngest son, who has autism and remains unaware of his sister’s death. Onyx’s aunt, Debbie Duran, shared that the family is devastated and is raising funds to cover funeral expenses, as they plan a celebration of life to honor Onyx’s memory.
Cornish’s preliminary hearing is set for September 3, 2025, at 8:30 a.m., as the investigation continues. The Caldwell Police Department, supported by the Canyon County Sheriff’s Office Crime Scene Lab, is meticulously processing the scene to uncover further details. For now, the community mourns the loss of a young woman whose light was extinguished too soon, while wrestling with the haunting question of how a father’s love could turn so tragically dark.
