The 1960s were a brutal decade. “Psycho,” the 1960 slasher thriller that jump-started the era, was a film that ripped a knife through everything the movies had led us to expect or believe. The JFK assassination was a cataclysm that seemed to cleave the century in half. In 1966, the same year the shocking murder of the Clutter family was immortalized in “In Cold Blood,” the slaying of eight student nurses by Richard Speck injected a new kind of monster into the American imagination. Two weeks after that, Charles Whitman climbed to the balcony of the University of Texas Tower and became the first mass shooter.

Yet that convulsive litany of killing, filtered through the new mindset of the media age, all seemed to be setting the stage for Charles Manson.

By the time Manson came along with his scraggly cult of sociopathic hippie wastrels, unleashing the two darkest nights of violence America had ever known, the Manson family seemed to be taking for granted that their most unspeakable acts would be immortalized, refracted through a media hall of mirrors, and that those acts would therefore invade our consciousness in a vicious new way. The violence that had ruled the ’60s, as much as the peace and love that (supposedly) reigned for about two years, had all been filtered through popular culture. But it wasn’t until Manson that the most chilling acts of savagery seemed to have been devised and executed — seemed to have been dreamed — as a kind of slaughterhouse theater. The Manson murders were inside-the-living-room terrorism on an apocalyptic scale.

Manson, who died Sunday at 83, has haunted the popular imagination ever since, and let’s be frank about why: He was one of the most hypnotic characters of the 20th century. He was a despicable monster, but he became a figment of larger-than-life fascination — the closest we were probably going to get to seeing the face of evil. On that score, he is matched, perhaps, only by Adolf Hitler; that’s the kind of mythic hold Manson had on us. And part of it is that he never stopped playing the role.

The derelict snarl; the long hair and beard and glittery-eyed mesmerist’s stare that gave him the look of a psychotic Antichrist rock star; the swastika carved into his forehead; the cosmic existential gutter-rat “philosophy” that he peddled like a wackadoo snake charmer. He was a vagrant, a killer, a guru, a sicko celebrity, a figure of punk fan worship, and a man who orchestrated his image even from prison, where he had a sixth sense for how to fashion his interviews into shrewdly taunting events of nuthouse theater.

It’s a startling and still counterintuitive fact that Manson, at least on the two nights (August 9 and 10, 1969) that he’s most famous for, didn’t actually kill anyone; he had others do it for him. But part of what makes that fact so terrifying is that, in delegating the most hideous violence to people he’d drugged and brainwashed, goaded and manipulated, he was like the sinister-cult-leader version of someone who set out to stage the ultimate horror movie — ultimate, in this case, meaning real.

Manson himself remained offstage, “directing” his “actors,” pushing them toward a catharsis of mayhem. The Manson murders were orchestrated to be a kind of show (the words scrawled in blood messages to the world), and the fact that one of the victims turned out to be Sharon Tate, the actress wife of Roman Polanski, who was eight months pregnant with their child, is part of what gave the tragedy its grisly karma. The killers didn’t know Sharon Tate was going to be there, but in the popular imagination it was as if Manson, at that moment, was attacking the Hollywood dream factory and replacing it with his own nightmare snuff factory.

That’s why what happened on those two nights changed our world. It shredded a piece of our souls, tearing away the last veil of safety, civilization, sanity. The slaughter sent out a signal: The old rules are gone. And it wasn’t just that this could now, theoretically, happen to anyone. (That was already the message of “In Cold Blood,” with its Kansas family executed out of nowhere.) It was that the extremity of the Manson murders, their quality of being a frenzied blood orgy emerging from the unhinged freedom of the ’60s, made a statement, and it was this: God has left the building. That an event this obscene could have happened at all was proof that He was no longer protecting us. That was the lingering and unsettling blasphemy of the Manson murders.

The birth of our 24/7 tabloid-news culture probably took place during the Patty Hearst kidnapping, with those cameras planted for months on the lawn of the Hearst estate, but in spirit it was really the Manson trial that launched it. Manson himself, reveling in his megalomaniac bad vibes, became an ominous reality star of staggering proportions. And what the trial revealed is that one of the reasons the Tate-LaBianca murders poured into the popular imagination is that their violence had already been baptized — in Manson’s acid-trip mind — in pop culture.

The fact that he heard personalized messages in Beatles songs sounded, of course, like the essence of schizophrenia. But then, hearing cosmic messages in pop songs was part of the essence of the ’60s. The magic of the Beatles is that they were speaking — directly — to each and every one of us. Manson’s reading of “Helter Skelter” hinged on the notion that if the Beatles, of all people, could create a song this scary and clashing and nihilistic, then we were heading for hell. And maybe, on some level, he was right.

Then again, Charles Manson was also a charlatan, a street-smart head case who may have believed his own cosmic hogwash, but who also used it to get people to do his bidding. His most notorious prison interview is the one he did in 1981 with Tom Snyder (“Get off the space shuttle, Charles!”), but his most revealing interview is the one he gave to Charlie Rose on “Nightwatch” in 1986, where he described, with low-key sincerity, how once he was let out of prison in 1967, his overwhelming priority was to stay out. Gathering and manipulating the members of the Family — getting them, wherever possible, to do his dirty work — became a key part of that strategy.

Who knows, in the end, what Charles Manson really believed? He believed in his own psychotic superiority, and as long as he was around, his presence linked us to the horror of what he did: his use of murder to assassinate the rationality of our society. He was bat-house crazy, but in a horrible way it worked. Now that he’s gone, we can all hope that he’s taking that karma back with him.