The Manson murders — the seven killings committed by Charles Manson’s followers in two days in Los Angeles in August 1969 — are often thought to mark the end of the 1960s, as if those brutal slayings were the inevitable outgrowth of the counterculture, the dark consequence of long hair, free love, casual drug use and a general breakdown of authority and social norms. This sentiment was most famously expressed by Joan Didion in her book “The White Album.” She wrote that “in a sense” it was true that “the Sixties ended abruptly on August 9, 1969, ended at the...