In 1969 The Rolling Stones hired the Hells Angels to help out at a free concert at the Altamont Speedway near Oakland, California. The Stones paid the Angels by giving the $500 worth of beer. One of the Hells Angels ended up killing an 18-year-old man. From Wikipedia:
Meredith Curly Hunter, Jr. (October 24, 1951 – December 6, 1969) was an 18-year-old African-American man who was killed at the 1969 Altamont Free Concert. During the performance by The Rolling Stones, Hunter approached the stage, and was violently driven off by members of the Hells Angels motorcycle club who had been contracted to serve as security guards. He subsequently returned to the stage area, drew a revolver, and was stabbed and beaten to death by Hells Angel Alan Passaro.
In the above video, Stones drummer Charlie Watts and Mick Jagger listen to Hells Angel leader Sonny Barger give his side of the story.
And in this video, Mick Jagger reviews a film of the killing: