In real life,O'Banion was supposed to be an inveterate practical joker with a sadistic slant. An innocent gag for him would be to give a guy Ex-Lax and tell him it was chocolate. The shooting bet that he made in the episode was based on his penchant for secretly packing both barrels of a shotgun with clay and betting his victims that they couldn't hit a particular target.
Supposedly,he would fall down laughing when the barrel exploded and the victim lost an eye, or hand, or part of an arm.

On a side note isn't it funny that an extremely narcissistic character is named Narcisse?