Joe Chill (real name Joseph Chilton) is the murderer of young Bruce Wayne's parents, thus making him indirectly responsible for Batman's existence.
History
Bruce Wayne’s parents were shot by a random street mugger, while trying to take their money and jewelry. He panicked and runs away when Bruce begins crying and calling for help.
Years later, Batman discovers that Joe Chill, the small-time crime boss he is investigating, is none other than the man who killed his parents. Batman reveals his identity to Joe, and his henchmen walked away when they learned that their boss' actions caused the existence of the hated Batman.
Eventually, Batman learns that Joe Chill was not a mere mugger, but actually a hitman who murdered the Waynes on orders from a Mafia boss named Lew Moxon, who held a personal grudge against Thomas Wayne for his incriminating testimony against him.
Alfred Pennyworth reminisces that Joe Chill is the son of Alice Chilton, one-time caretaker of young Bruce Wayne.