Bob was one of The Joker's henchmen. They seemed to have a fairly amicable relationship, however on the night that The Joker decided to kill everyone in Gotham City with his Smilex gas, Batman interfeared with his plan and got rid of the huge balloons that The Joker was storing the gas in. Upset that Batman had done this, The Joker shot Bob dead in cold blood out of sheer frustration.