In the Prohibition Era, Bugsy Siegel was notable for a lot of things. One of them was as the public face of the lesser-known Jewish Mafia, and as a hugely influential figure within the National Crime Syndicate that included Al Capone, Lucky Luciano, Frank Costello, and probably every other '30s gangster you've ever heard of. But apart from being a racketeer and bootlegger, Siegel's primary job was as a contract killer and head of Murder Inc. Murder Inc. was the enforcement arm of the American Mafia, and (headed by Siegel) was responsible for at least 1,400 murders and contract killings. Bugsy was, in his time, effectively the King of All Hitmen.
No one knows exactly how many murders Bugsy himself committed, but his personal body count could very well have been in the hundreds. Bugsy was himself killed by another hitman in 1947, most likely over gambling debts owed to his former employers.