Sam Giancana was a Chicago mob boss. In his teens he was already a notorious triggerman and by twenty he was the prime suspect in three murder investigations, including one of a candidate for ward committee. The WWII draft board declared him 4F (unfit for service) not for physical health, but because he said he was a psychopath. By the 50s, he was running the show.
But this success brought intense FBI scrutiny. He used Frank Sinatra to try to get the Kennedys off his back, but when Sinatra failed to deliver (he couldn't "get change for a quarter" from the Kennedys, the gangster said), he even contemplated having Sinatra killed. After ending up in prison again in 1965, he fled to Mexico and was forced out of the boss' chair. Mexico kicked him out in 1974 and back in the US he was repeatedly forced to testify to a federal grand jury. On June 19, 1975, shortly before yet another grand jury appearance, he was cooking Italian sausage in his basement kitchen. The housekeeper found his body there. He had been shot seven times, once in the back of the head and six times around the mouth - the traditional way to shoot a squealer. The murder remains unsolved.