Scott Marcello was supposed to be the man to redeem KPMG LLP’s audit business. Instead, he and other top partners became the center of a scandal that tarnished the firm’s reputation.

The accounting world was stunned last week when Mr. Marcello, KPMG’s top audit official, was fired over a leak of confidential information. The firm said Mr. Marcello, who turns 54 this month, and four other partners were let go over the mishandling of a tip that gave the firm improper advance word about which of its audits its regulator planned...