Quiet, loyal Bob Hawkins, otherwise known as "Bob the Goon" is the Joker's goon, and throughout the film has unquestioningly obeyed whatever order the Joker gives him. When Batman foils the Joker's nefarious scheme to poison Gotham City with Smylex gas from parade balloons, the Joker needs someone to take out his frustration on, and poor Bob is the only person handy at the time.
"Bob? Gun," the Joker says, motioning with his hand.
Obediently, Bob takes out his pistol and hands it to the Joker, who immediately shoots Bob with it. Bob flies back out of frame, and the Joker hands the gun to another goon, explaining, "I'm gonna need a minute or two alone, boys."