Gaspipe Casso killed more than a few.
I'm interested in the older Dons. Genovese definitely was a killer, but I've never read anything about direct killing by Bonanno, Costello, Luciano, Lucchese or Mangano. Doesn't mean they didn't kill, just that I haven't read about it.
Capone, not a Mafia don, killed twice before he was 21, precipitating his flight to Chicago. There he killed with his own hands only once--a small timer who'd slapped around his bosom buddy Jake Guzik. He baseball-batted Scalise, Anselmi and Giunta, but they were finished off by Accardo and McGurn.
I cant remember where I read it but IIRC Gambino said Lucchese was one of the best hit men he ever knew, even better than Anastasia because he was a marksman and always kept his cool never letting anger or emotion cloud his thoughts.
