|Midnight (Call It Murder) (1934) [Garboni]: Shot to death (off-screen) by Sidney Fox.
Bullets or Ballots (1936) [Nick 'Bugs' Fenner]: Shot to death in a shoot-out with Edward G. Robinson. (Thanks to Paul)
The Petrified Forest (1936) [Duke Mantee]: Shot to death (off-screen) by police, after Humphrey shoots Leslie Howard. (Thanks to MP)
Dead End (1937) [Hugh 'Baby Face' Martin]: Shot to death by police when he opens fire on them, after first being shot by Joel McCrea and falling from a fire escape.
The Amazing Dr. Clitterhouse (1938) ['Rocks' Valentine]: Poisoned when Edward G. Robinson spikes his liquor, after Humphrey tries to blackmail Edward. (Thanks to Stephen)
Angels with Dirty Faces (1938) [James Frazier]: Shot in the back by James Cagney when Humphrey tries to flee from the office.
King of the Underworld (1939) [Joe Gurney]: Shot to death in a shoot-out with federal agents. (Thanks to Paul)
The Oklahoma Kid (1939) [Whip McCord]: Shot to death by the mortally-wounded Harvey Stephens, just as Humphrey is about to kill James Cagney.
The Return of Doctor X (1939) [Dr. Maurice Xavier a.k.a. Marshall Quesne]: Shot to death by police when they break into his laboratory to rescue Rosemary Lane.
The Roaring Twenties (1939) [George Hally]: Shot three times by James Cagney in Humphrey's office; we only see Humphrey fall after the first (non-fatal) shot, followed by a shot of James firing the last two shots. Humphrey's body is not shown afterwards. (Thanks to Mac)
Virginia City (1940) [John Murrell]: Shot to death in a shoot-out between his bandits and Union soldiers. (Thanks to Paul)
High Sierra (1941) [Roy 'Mad Dog' Earle]: Shot to death by a police sharpshooter when he emerges from his hiding spot on the mountainside to call down to Ida Lupino. (Thanks to Mac)
The Wagons Roll at Night (1941) [Nick Coster]: Mauled to death by a lion after he draws its attention away from Eddie Albert. (Thanks to Paul)
The Big Shot (1942) [Joseph 'Duke' Berne]: Shot in a shoot-out with Stanley Ridges; he dies in the prison hospital shortly afterwards. (Thanks to Paul)
Passage to Marseille (1944) [Jean Matrac]: Killed when his plane is shot down by German fighters. (Thanks to Paul)
The Treasure of the Sierra Madre (1948) [Fred C. Dobbs]: Hacked to death with a machete by Alfonso Bedoya. We only see the blade coming down before the scene cuts away.
Sirocco (1951) [Harry Smith]: Killed in an explosion in an alley. (Thanks to Fred)
The Desperate Hours (1955) [Glenn Griffin]: Machine-gunned by police when he commits "suicide-by-cop" by throwing an empty gun at a police spotlight. (See also Mickey Rourke in the 1990 version.) (Thanks to Mac)
Tales from the Crypt: You, Murderer (1995) [Lou Spinelli]: Beaten to death with a statue by Isabella Rossellini. (Although this episode was made 38 years after Humphrey's real-life death, he "guest-starred" through edited footage from previous films, with new dialogue dubbed in by Robert Sacchi.) (Thanks to Andrew)