Eric Hainstock was born on April 4, 1991, in La Crosse, Wisconsin, to Shawn Hainstock and Lisa Marie Buttke. His parents divorced when he was two years old. In December 1995, they went to court to seek custody of Eric. When he was nine, a court terminated his mother’s parental rights because she had not paid child support.

Eric lived with his father and stepmother in a two-story A-frame farmhouse about four miles northwest of Cazenovia, Wisconsin. His father was unemployed and received disability pensions. After experiencing difficulties in his home life, Eric was forced to live with his paternal grandmother due to allegations of mistreatment by his father. He returned to his father's care in April 2002.

Eric claimed that he was treated harshly by his father. In a letter, he described doing all of the cleaning in their home, sometimes until past midnight. He said his father disciplined him severely, making him stand in a corner for long periods. This treatment contributed to a negative self-image.

At Weston High School, Eric faced bullying from other students. He claimed around 25 to 30 classmates picked on him. He reported that when he complained to teachers, they did not take action to help him. Despite having behavior problems and poor test scores at his previous school, Eric transferred to Weston High School in the sixth grade.

During his time in high school, Eric experienced a physical confrontation with his stepmother. Following this incident, he had visible bite marks on his body. He struggled with hygiene and received limited time to shower, which affected how he felt about himself.

In addition to these issues, Eric was diagnosed with attention deficit hyperactivity disorder (ADHD) and was treated with prescription drugs. Reports suggest that these medications may have caused emotional imbalances that affected his behavior. A psychologist who evaluated Eric later diagnosed him with ADHD, depression, and features of borderline personality disorder after reviewing his history and spending time with him.

On September 29, 2006, Eric Hainstock entered Weston High School in Cazenovia, Wisconsin, with two firearms: a .22 caliber revolver and a 20-gauge shotgun. He took these guns from his father's locked cabinet. Around 8:00 a.m., he aimed the shotgun at a social studies teacher. The school custodian, Dave Thompson, noticed him and wrestled the shotgun away from Hainstock. As this was happening, Principal John Alfred Klang confronted Hainstock, who still had the revolver in his possession. Hainstock pulled the revolver from his jacket and fired several shots.

During the struggle, Klang tackled Hainstock, trying to take control of the situation. The scene became violent, and Klang was shot. Staff and students helped to subdue Hainstock until the police arrived. Klang was severely injured and received treatment at Reedsburg Area Medical Center. He later underwent surgery but died at the University of Wisconsin Hospital in Madison in the afternoon. He was the only person shot in this incident.

In police interviews, Hainstock claimed that he had not intended to kill Klang. He insisted that his goal was to force the principal to listen to his complaints about relentless bullying and mistreatment. However, his actions and the deadly outcome ensured that the legal consequences would be severe.

Eric Hainstock was immediately arrested and charged with first-degree intentional homicide. His trial began in Sauk County Circuit Court, where prosecutors argued that his actions were deliberate and premeditated. They cited his decision to bring two firearms to school and his history of planning confrontations with authority figures. The defense, however, presented evidence of Hainstock’s traumatic upbringing, history of abuse, bullying, and mental health struggles, arguing that these factors had driven him to desperation. 

On August 2, 2007, he was found guilty and sentenced to life imprisonment. He will be eligible for parole in 2037 when he is 46 years old. Hainstock is currently serving his sentence at Oshkosh Correctional Institution after being moved from the Green Bay Correctional Institution.