On November 3, 1924, Dean O'Banion inadvertently signed his own death warrant during an argumentative phone call to arch-rival Angelo Genna. Their disagreement originated at The Ship, the gambling casino that the North Side gang boss owned along with the Torrio Syndicate. On this day, O'Banion sat in with Al Capone, Frank Nitti, Frank Rio, and others to tally the week's profits. It was mentioned that Angelo Genna had dropped a large amount of cash, plus a sizable marker. Capone recommended that they cancel the marker as a professional courtesy. O'Banion, instead, got Genna on the telephone and demanded that he pay his debt within a week. With this personal insult, Angelo Genna and his family could no longer be restrained.[5] Until then, Mike Merlo and the Unione Siciliana had refused to sanction a hit on O'Banion. However, Merlo had terminal cancer and died on November 8, 1924. With Merlo gone, the Gennas and South Siders were free to move on O'Banion.
Using the Merlo funeral as a cover story, over the next few days Brooklyn gangster Frankie Yale and others visited Schofield's, O'Banion's flower shop, to discuss floral arrangements. However, the real purpose of these visits was to memorize the store layout for the hit on O'Banion.
On the morning of November 10, 1924, O'Banion was clipping chrysanthemums in Schofield's back room. Yale entered the shop with Genna gunmen John Scalise and Albert Anselmi. When O'Banion and Yale shook hands, Yale grasped O'Banion's hand in a tight grip. At the same time, Scalise and Anselmi stepped aside and fired two bullets into O'Banion's chest and two into his throat. One of the killers fired a final shot into the back of his head as he lay face down on the floor.