Oswald was killed to keep him from talking. The mob frames Oswald for the assassination, uses his gun, gets him a job in the DSBD, then they kill him. Case closed. Oswald was seen in the company of Carlos Marcello at least two times and the word on the street in New Orleans was that "friends" had JFK assassinated.
Carlos Marcello and Santo Trafficante conspired to kill JFK. There have been several instances where Marcello admitted having JFK killed. One time was in a prison cell where Marcello told his cell mate, and the FBI had the cell bugged or the cellmate was working for the FBI. There are one or two other times where Marcello made mention of them hitting JFK. Trafficante told his lawyer, Frank Ragano, on his deathbed that they did kill JFK.