As far as Ricca goes, in the old country, he killed a man named Emilio Parrillo supposedly for breaking an engagement to Ricca's sister. In reality Ricca killed Parrillo on Mafia orders. Ricca then killed Vincenzo Capasso, who had testified against him in the Parillo trial, by slitting his throat.
Bugsy Siegel was a prolific killer, and besides being suspected in the deaths of Maranzano and Masseria, it is beyond resonable doubt that he killed all three of the Fabrizzo brothers. I believe he was also one of the early Murder,Inc guys.