On December 6, 1969 the Rolling Stones organized - if that's the right word - and headlined the free Altamont Music Festival at the Altamont Speedway in northern California. This was a few months after Woodstock, and there was a general idea that the Stones would have their own Woodstock.
It had been scheduled to happen at the Golden Gate Park, but San Francisco officials revoked their permission (if they even had proper paperwork to begin with) at the last minute when Mick Jagger publicly announced that the Stones would be there without having taken any crowd control precautions. It was then supposed to be going to Sears Point Raceway, but that fell through as well. The Altamont venue was finally settled on as late as December 4.
Surprisingly enough, 400,000 rock fans showing up at a place selected within the last 48 hours turned out badly. Four people managed to wind up dead. A couple of people got killed in an auto accident amidst the chaos, and one person managed to drown in a drainage ditch. The famous death, of course, was an 18-year-old kid named Meredith Hunter, who was stabbed multiple times and kicked to death on film right in front of the stage by Hell's Angels during the Rolling Stones set.
This is just the kind of thing that BEGS for pontification, asserting Big Profound Meaning and so forth. It was the anti-Woodstock, and the dark side of the hippie culture, the death of the '60s, etc. Among other things, the event was immortalized in Don McLean's famous "American Pie" wherein the Satanic Jack Flash was lustfully presiding over the "sacrificial rite." It was the climactic destruction and sweeping away of Buddy Holly's rock and roll "true love ways." It was the final day when the music really, really died.
Obviously, a lot of this stuff is being loaded onto happenstance, which leads to people twisting the actual random chaos of the event to fit their mythologies. For example, it has sometimes been claimed that Hunter was killed while the Stones were playing "Sympathy for the Devil." Obviously that would be perfect mythology, with the whole Satanic "sacrificial rite" angle. In fact, they had finished that song, and were specifically playing "Under My Thumb" at the critical moment — which to me is a more interesting sentiment to be playing out for that event.
Fortuitously, the Rolling Stones had a camera crew filming their tour for a movie, which was eventually released under the title Gimme Shelter.
It was directed by David and Albert Maysles. Among several others, young George Lucas was working as a cameraman. A piece of video isn't the same as being there, and you also have to consider all the stuff that was going on that did NOT end up on film. Still, the camera doesn't lie, and it captured a lot of fascinating things going on amongst this third of a million people.