Bernadette Protti grew up in the comfortable suburban town of Orinda, California, during the early 1980s. She was one of several children in a middle‑class Catholic family known locally for being quiet and hard‑working. Her parents were described as caring but strict, and the family emphasized academic performance, modesty, and community reputation.

At Miramonte High School, Protti was a good student, polite and soft‑spoken, though not considered part of the popular crowd. She participated in extracurricular activities but reportedly struggled with feelings of social inadequacy and envy toward more outgoing classmates. Among these was Kirsten Marina Costas, an athletic and well‑liked student involved in the cheerleading squad and swimming team.

Witness accounts and later psychological assessments suggested that Bernadette admired Kirsten’s confidence and social status, but over time her admiration evolved into resentment and fixation. Classmates later recalled that Bernadette had expressed feelings of rejection and humiliation when she failed to be accepted into certain student social groups that included Kirsten.

Despite appearing shy and ordinary to most peers, Bernadette quietly harbored insecurity about her social standing. This emotional conflict would culminate in one of California’s most infamous teenage murders.

On June 23, 1984, Bernadette Protti devised a plan to win Kirsten Costas's friendship by inviting her to a fabricated dinner event for the Bob-o-Links, a sorority-like group at their high school. Protti called Costas's mother, posing as someone organizing the dinner, and instructed her to keep it a secret from Kirsten but ensure she was ready for the event. Believing the invitation to be genuine, Costas agreed to attend.​

Protti picked up Costas from her home under the pretense of attending the dinner. During their drive, Protti revealed that there was no dinner, which led to an argument between the two. Feeling deceived and upset, Costas demanded to be taken home. Instead, Protti drove her to an unfamiliar neighborhood, where Costas exited the vehicle and sought help from a nearby residence. She reached the home of Alex and Mary Jane Arnold, explaining that her friend had gone "weird." Unable to contact her parents, the Arnolds offered to drive her home. As they approached Costas's house, they noticed Protti following them in her family's Pinto.

Upon arriving near her home, Costas exited the Arnold's vehicle. At that moment, Protti confronted her and, in a sudden act of violence, stabbed Costas five times with a kitchen knife. Protti then fled the scene. Despite her injuries, Costas managed to reach another neighbor's house, where she collapsed. Emergency services were called, but Costas was pronounced dead shortly after arriving at the hospital.

The investigation into Costas's murder lasted nearly six months. Initially, Protti passed a lie-detector test, and her alibi went unchecked. However, inconsistencies in her story led authorities to re-examine her statements. After being informed by an FBI agent that her arrest was imminent, Protti confessed to her mother in a written letter, detailing her involvement in the crime. She was subsequently arrested and charged with second-degree murder.

In March 1985, Protti was convicted and sentenced to a maximum of nine years in the California Youth Authority. She was released on parole in 1992 after serving seven years. The case garnered significant media attention and has been the subject of various adaptations, including the 1994 television movie "A Friend to Die For" and its 2019 remake, "Death of a Cheerleader."