At some point, Boss Grissom discovered that his top lieutenant Jack Napier was having an affair with his mistress Alicia. His plans for revenge against Jack coincided with the DA Harvey Dent's attempts to connect him with Axis Chemicals, and so Grissom orders Jack to lead a group to the chemical plant to seize the evidence left there before the cops find it. Grissom then calls Max Eckhardt, his inside man with the police, and orders him to arrest Napier.
When Eckhardt and his squad raid Axis, the lieutenant is killed by Napier and Napier himself is confronted by Batman. After being injured and falling into a vat of chemical waste, Napier is assumed dead, but the crazed gangster survives and is reborn as the Joker.