He wasn't "set up", but he was probably not guilty, in fact a jury ultimately found him not guilty -- the press' speculation led to his indictment.
From a short biography site:
http://www.2020site.org/fattyarbuckle/bio.htmlDuring the first eight months of 1921 Roscoe made nine feature films for Paramount. By September he was in desperate need of a break. On Labor Day Weekend he decided to take a break in San Francisco along with a couple of friends. They arrived at the St. Francis Hotel and immediately ordered whiskey and a victrola delivered to their room. For the next two days a period of drinking, partying and alleged debauchery followed. Sometime during this period the party was joined by a dress model named Bambina Maude Delmont, an agent named Al Semnacher and a young actress by the name of Virginia Rappe. Rappe began to complain about abdominal pains, and went to the bathroom attached to Arbuckle's sweet (
sic) to vomit. From here, the details become very unclear. One possibility is that Arbuckle found her in his bathroom, brought her to his bed and applied ice to her abdomen. She didn't go to the hospital for three more days and died that Friday, at the hospital, of a ruptured bladder.
On September 10 Roscoe ‘Fatty’ Arbuckle was charged with murder. Immediately the press was all over the story and sensationalized that Arbuckle had caused the rupture by crushing Rappe while on top of her. Even more lurid accounts suggested that Fatty Arbuckle had violated her Virginia (
sic) using a champagne bottle. Two days later voluntary and state mandated bans were imposed on Arbuckle’s movies. On September 13, a Grand Jury returned an indictment of manslaughter against Arbuckle.
On November 18 the court case of the century began. Three weeks later the Judge dismissed the jury after they were unable to come to a decision. A new trial was set for January, 1922. Again the jury failed to reach agreement and a mistrial was once again entered into the records. The third trial began in March, 1922. This time after just six minutes of deliberation the jury reached agreement. Roscoe Arbuckle was found not guilty of manslaughter.