Towards the final scene of Seven Psychopaths, Billy is holding Bonny with the flare gun to its head. Marty initially refuses to go away. When he approaches Billy, he shows Marty something which causes Marty to run to the car and leave. What is it that Billy shows him that convinces Marty to leave?
Billy is holding a Jack of Diamonds in his own hand. You can see it better after he is shot and lying on the ground, and is asking Bonny for his paw.