This Gun for Hire (1942)

Hit man Philip Raven, who's kind to children and cats, kills a blackmailer and is paid off by traitor Willard Gates in "hot" money. Meanwhile, pert entertainer Ellen Graham, girlfriend of police Lieut. Crane (who's after Raven) is enlisted by a Senate committee to help investigate Gates. Raven, seeking Gates for revenge, meets Ellen on the train; their relationship gradually evolves from that of killer and potential victim to an uneasy alliance against a common enemy.