On August 27, 1964, Kemper's grandmother, Maude Matilda Hughey Kemper (1897-1964), was sitting at the kitchen table when she and Kemper had an argument. Enraged by the argument, Kemper stormed off and grabbed the .22 caliber rifle which his grandfather had given him for hunting. He then returned to the kitchen and, when Maude told him not to shoot any birds, fatally shot her in the head before firing twice more into her back. Some accounts allege that Maude Kemper additionally suffered multiple post-mortem stab wounds with a kitchen knife. He then dragged her body out of the kitchen and into her bedroom. When Kemper's grandfather, Edmund Emil Kemper (1892-1964), came home from grocery shopping, Kemper went outside and fatally shot him in the driveway. He was unsure of what to do next and so phoned his mother, who urged him to contact the local police. Kemper then called the police and waited for them to take him into custody.
When questioned by authorities, Kemper said that he "just wanted to see what it felt like to kill Grandma," and that he killed his grandfather so that he would not have to find out that his wife was dead. Psychiatrist Donald Lunde, who interviewed Kemper at length during adulthood, wrote that with these murders, "In his way, [Kemper] had avenged the rejection of both his father and his mother." Kemper's crimes were deemed incomprehensible for a 15-year-old to commit, and court psychiatrists diagnosed him as suffering from paranoid schizophrenia before sending him to the criminally insane unit of the Atascadero State Hospital. He was later released.