Manson didn't care about being officially listed as cowriter. Instead, he took some money and a motorcycle and other undisclosed items (about $100,000 worth of stuff according to Wilson) as payment for the song. He became enraged, however, when he heard that his lyrics had been altered. This led to him delivering a bullet to Wilson as a threat.
Some have theorized that this incident, combined with rejection by the music industry, caused Manson to lash out with the Family murders, but that's just one of about half-a-dozen theories in a convoluted mix of possibilities that seems to only become more uncertain as the years pass. The official theory put forth by prosecutor Vincent Bugliosi, and the one accepted in the court of law at Manson's trial, was that the murders were done in order to start a race war as part of an insane end-of-the world fantasy inspired by the Beatles' White Album, particularly the song "Helter Skelter
Whatever the case, Wilson was badly shaken after hearing about the Manson Family murders. Mark Dillon, author of Fifty Sides of the Beach Boys: The Songs That Tell Their Story
, has written that some close to Wilson believe that this incident pushed him over the edge into a spiral of self-destruction. Consumed with guilt for his role in bringing Manson into the entertainment industry and getting Sharon Tate killed, his already considerable drug and alcohol abuse got far worse.
So, there it is, the dark story hiding behind a vacuous love song. Remember as you listen that murder and mayhem lurk in the shadows.