By February 1969, Mullin seemed obsessed with Eastern religions, his family noting that he had become "more and more unrealistic" in daily behavior. A month later, they persuaded him to enter a mental institution, but he refused to cooperate with psychiatrists and was released after six weeks. October found him in the depths of full-blown paranoid schizophrenia, exacerbated by continuing use of marijuana and LSD. Mullin heard "voices," commanding him to shave his head or burn his penis with a cigarette, and he obeyed their every order. Briefly returned to the hospital, he began writing letters to dozens of total strangers, signing them, "a human sacrifice, Herb Mullin." An ill-advised visit to Hawaii, in June 1970, resulted in Mullin's commitment to a mental institution there. Back in Santa Cruz, his odd behavior led to conflicts with police, and his problems were not erased by fifteen months of hiding out in cheap San Francisco hotels. By the time he came home again, in September 1972, the disembodied voices were commanding him to kill.

On October 13, 1972, while driving aimlessly through the Santa Cruz mountains, Mullin spotted elderly transient Lawrence White. Pulling his car to the side of the road, Mullin asked White to help him with some "engine trouble," then beat the old man to death with a baseball bat and left his body where it lay. Eleven days later, he picked up co-ed Mary Guilfoyle, stabbed her in the heart, then disemboweled her, scattering her organs on the shoulder of a lonely road, where skeletal remains were found in February 1973. On November 2, Mullin spoke too freely in the confessional at St. Mary's Church, fatally stabbing Father Henry Tomei in a bid to protect himself from exposure.

Mullin's crimes coincidentally overlapped with those of serial slayer Edmund Kemper, earning Santa Cruz an unwelcome reputation as "Murderville, USA." By November 1972, Herbert was hearing brand- new voices, emanating from prospective victims, begging him to kill them. He bought a pistol in December and resumed the hunt.

On January 25, 1973, Mullin went looking for Jim Gianera, the man who had "turned him on" to marijuana years earlier. Herb now regarded that act as part of a plot to destroy his mind, and he meant to avenge himself. Calling at Gianera's old address, he received new directions from 29-year-old Kathy Francis. Moving on, he found Gianera at home, shot the man to death, then knifed and shot Gianera's wife for good measure. From there, Mullin doubled back to kill Kathy Francis and her two small sons, shooting all three as they lay in bed.

On February 6, Mullin was hiking in a nearby state park, when he met four teenage campers. Approaching the boys with casual conversation, he whipped out his gun and killed all four in a burst of rapid fire, before they could react or flee. A week later, driving through Santa Cruz, Mullin pulled to the curb and fatally shot Fred Perez, while the old man was working in his garden. This time, neighbors saw his license plate, and Mullin was arrested by patrolmen moments later.

In custody, Mullin confessed to his crimes, maintaining that the homicides were necessary to prevent catastrophic earthquakes from destroying California. Charged and convicted in ten of the murders (omitting White, Guilfoyle, and Tomei), Mullin was sentenced to life imprisonment. He will be eligible for parole in the year 2020 A.D.