When JFK was elected president, he appointed his brother, Robert Kennedy, to Attorney General. Immediately Robert began attacking leaders of the mafia, which angered many large crime families. Specifically, Robert Kennedy targeted Jimmy Hoffa, who was the elected leader of the union and was suspected to work with multiple large mob leaders. During this time, mafias controlled many labor unions in big cities. The mafia could have assassinated JFK as an act of revenge against Robert Kennedy. In order to cause the most grief, they assassinated President Kennedy. Three different mobs claimed responsibility for Kennedy's death immediately after the assassination.
Again, there is no evidence that supports this conspiracy theory. The mafia was angry with Robert so they would not be targeting JFK. The mob had many ties to the Kennedy family, and correspondingly had political power, so they would not be doing anything to jeopardize that relationship.
[P1] Robert Kennedy attacked organized crime when he was Attorney General. [P2] The mafia controlled most labor unions in big cities during this time. [P3] Therefore, the mafia assassinated JFK due to Robert Kennedy's attacks against organized crime.
Rejecting the premises
[Rejecting P3] There is no evidence that supports this theory.