Goodfellas is full of drama, betrayals, and mysteries, as is the death of Tommy DeVito, who according to a fan theory, was betrayed by his friend Jimmy Conway, thus leading to his death. Martin Scorsese is one of the most respected and popular filmmakers in the industry, and while he has explored a variety of genres throughout his career, he’s still best known for his gangster movies, of which Goodfellas is considered the best.
Based on the book Wiseguy by Nicholas Pileggi, Goodfellas chronicles the life of mob associate Henry Hill (Ray Liotta), from his days as a teenager fascinated by the mafia presence in his neighborhood, to his full involvement with the crime family of Paul Cicero (Paul Sorvino) and later his decision to become an FBI informant. Henry began working for the mob at the same time as Tommy DeVito (Joe Pesci), so they basically grew up together. While working for Paul and his crew, Henry and Tommy met Jimmy Conway (Robert De Niro), with whom they worked closely many times.
One of their most notable crimes together was the Lufthansa heist, planned by Jimmy and with Tommy as part of the crew, with Henry just being an observer. The three were so close that when Tommy killed Billy Batts, Jimmy helped him beat him up and the three later buried his body. Some time later, Tommy was on his way to become a “made man”, but it was a trap and he was murdered, presumably as retaliation for Batts' death. In the movie and in real life it’s unknown who told Batts’ people that Tommy was responsible, but a fan theory suggests it was someone very close to Tommy, who didn’t hesitate on killing those who could represent a threat: Jimmy Conway.
A fan theory posted on Reddit explains that Jimmy was the one who betrayed Tommy and that his reaction to his death was part of his act. The author explains that Henry mentions that, after the Lufthansa heist, Jimmy was a “nervous wreck” and became paranoid and more violent, as seen when he ordered the murder of many of the men involved in the heist just to make sure no one betrayed him. Jimmy even tried to kill Karen Hill at some point, so it wouldn’t be surprising if his paranoia took him to such a bad place where he would betray one of his closest friends to his benefit. The author adds that Jimmy might have told Paulie that Tommy killed Batts and omitted his involvement in it in order to protect himself, as if the Gambino family knew he was involved, he would have been killed too.
In Goodfellas, the man who pulled the trigger was Tuddy Cicero (Frank DiLeo), Paul’s brother, which connects with the explanation of what happened to the real Tommy (whose name was actually Tommy DeSimone): Paul told the Gambino family that Tommy killed Batts, this as revenge as he tried to rape Karen Hill (with whom Paul had an affair while Henry was in jail). Back to Goodfellas, and even if Paul was the one who told the Gambino family about it, the above theory could still work, as Jimmy could have told Paul what happened, but omitting a couple of details. Goodfellas left this part of Tommy’s story somewhat open to interpretation and is up to every viewer to come up with their own explanations.