"The Mob did it" theory is based on allegations that the Kennedy Administration "owed" the Mob for helping to elect him, and for assisting the CIA in his ridiculous plots to assassinate Castro. So, when brother Bobby took after the Mob with a vengeance, they retaliated by killing him. I don't believe it. The Mob (at least in those days) never assassinated public officials because doing so would bring more heat on them than they were getting. And if they did, why kill JFK--and leave ruthless brother Bobby to exact vengeance on them? Jack Ruby had casual contacts with Mob fringe people because anyone who operated a strip club in America at the time dealt with Mob associates who controlled the pipeline of strippers and other entertainers. But he had far, far more contacts with the Dallas police, who controlled his all-important liquor and cabaret licenses. And all of Ruby's arrests were related to his club: assaulting patrons and employees; failure to pay various taxes, license violations, etc. A Mob associate once approached him and his brother about a dope deal, but they turned the guy down.
Finally, ask one question: If the Mob really wanted to kill JFK, why would they use or involve a certifiable nut-case like Oswald, when they had plenty of trained assassins? And why would they use another certifiable nut-case like Ruby to kill Oswald, and leave him behind?