|Undersea Kingdom (1936) [Hakur]: Killed in an explosion (along with all the other bad guys) when the U.S. Navy fires on Monte Blue's tower; we only see the tower blowing up.
Of Mice and Men (1939) [Lennie Small]: Shot to death by Burgess Meredith.
Man Made Monster (1941) [Dan McCormick]: Dies after his insulated suit is torn on a fence, releasing the electricity that was keeping him alive. (Thanks to Dignan)
The Wolf Man (1941) [Larry Talbot]: Beaten to death (while in his wolf-man form) with a silver-handled cane by his father (Claude Rains) in the woods. His body changes back to his human form after death. (Note: He was brought back to life when some grave robbers opened his crypt in the 1943 sequel Frankenstein Meets the Wolf Man, but it's clear that the original filmmakers intended this as a genuine death scene.)
Son of Dracula (1943) [Count Alucard a.k.a. Count Dracula]: Dissolved into a skeleton by the light of the rising sun, after Robert Paige destroys Lon's coffin. (Thanks to Anton)
House of Frankenstein (1944) [Lawrence Talbot]: Shot in the chest (off-camera) with a silver bullet by Elena Verdugo, while he's in his wolf-man form; he staggers through the woods before collapsing, then resumes his human form after death. (Lon reappeared in the 1945 sequel House of Dracula, with no explanation for his resurrection.)
Pillow of Death (1945) [Wayne Fletcher]: Commits suicide by jumping from a window, after he hallucinates hearing Victoria Horne's voice commanding him to do so.
Only the Valiant (1951) [Trooper Kebussyan]: Shot to death by Apache warriors while Lon is covering Gregory Peck, as Gregory sets an explosive charge. (Thanks to Brian)
Raiders of the Seven Seas (1953) [Peg Leg]: Killed by Anthony Caruso (though I don't know the details); Anthony then places Donna Reed's pearls in Lon's hand to frame her for his murder.
Passion (1954) [Castro]: Stabbed to death in a fight with Cornel Wilde, after Lon pulls a hidden knife out of his boot.
The Indian Fighter (1955) [Chivington]: Killed in an explosion when the dynamite that he and Walter Matthau were planting goes off.
Not as a Stranger (1955) [Job Marsh]: Hit by a bus (off-screen); his body is shown afterwards being brought into the hospital on a stretcher when his son (Robert Mitchum) sees him.
The Black Sleep (Dr. Cadman's Secret) (1956) [Dr. Munroe a.k.a. Mungo]: Strangled with a chain by George Sawaya while Lon is struggling with Tor Johnson.
Indestructible Man (1956) [Charles 'Butcher' Benton]: Executed (off-screen) by the electric chair; his body is shown afterwards when Joe Flynn brings him to Robert Shayne's laboratory, where Robert brings him back to life. Lon is electrocuted again when he collides with the power lines at a construction site while trying to make a getaway in a stolen crane.
Wagon Train: The Jose Morales Story (1960) [Louis Roque]: Killed (along with Lee Marvin) in a battle with Mescalero warriors; their bodies are shown afterwards when Terry Wilson finds them. (Thanks to Gordon)
Spider Baby: Or, the Maddest Story Ever Told (The Liver Eaters) (1964) [Bruno, the Chauffeur]: Killed in an explosion (along with Jill Banner, Beverly Washburn, and everybody else in the house) when he sets off some dynamite to put them all out of their misery.
Welcome to Hard Times (Killer on a Horse) (1967) [Avery]: Hit on the head with a bottle by Aldo Ray in the saloon.
Buckskin (The Frontiersman) (1968) [Sheriff Tangley]: Shot to death by Barry Sullivan. (Thanks to Brian)
Dracula vs. Frankenstein (Blood of Frankenstein; The Blood Seekers; Satan's Bloody Freaks) (1971) [Groton]: Shot to death by police as Lon is chasing Regina Carrol on the beach.