|Circus of Horrors (Phantom of the Circus) (1960) [Vanet]: Crushed to death by the circus' bear after it steps on a broken bottle, sending it into a rage.
A Story of David: The Hunted (1961) [Nabal]: Dies of a heart attack during an argument with Barbara Shelley. (Thanks to Gordon)
The Great Escape (1963) [Flight Lt. Colin 'The Forger' Blythe]: Shot to death by German soldiers after his airplane crashes. (Thanks to Robert and Stephen)
The Outer Limits: The Man with the Power (1963) [Harold J. Finley]: Commits suicide by using his telekinetic powers to disintegrate himself. (Thanks to Stephen)
Dr. Crippen (1964) [Dr. Hawley Harvey Crippen]: Executed by hanging. (Thanks to Gordon)
Fantastic Voyage (Microscopia; Strange Journey) (1966) [Dr. Michaels]: Suffocated/devoured by a white corpuscle after being shrunk to microscopic size and sent into Jean Del Val's bloodstream. (Thanks to Stephen)
Will Penny (1968) [Preacher Quint]: Shot to death by Charlton Heston. (Thanks to Mac)
Kidnapped (1971) [Ebenezer Balfour]: Dies of heart failure in bed. (Thanks to Brian)
Henry VIII and His Six Wives (1973) [Thomas Cromwell]: Executed (off-screen) by beheading; we only see him being led away. (Thanks to Johan)
Barry McKenzie Holds His Own (1974) [Erich, Count von Plasma]: Turned into a skeleton when Barry Crocker confronts him with a makeshift cross made from beer cans. (Played for comic effect.)
The Mutations (The Freakmaker) (1974) [Professor Nolter]: Partially devoured by Scott Antony after Donald transforms Scott into a "human venus flytrap." His skinless body (an obvious special-effects prop) is shown afterwards. (Thanks to Kyle)
The Count of Monte Cristo (1975) [Danglars]: Commits suicide (off-camera) by shooting himself in the head; we only see him preparing to shoot, then hear the sound of the gunshot from the next room. His body (just his hand) is shown afterwards when his assistant rushes into the room. (Thanks to Johan)
I Don't Want to Be Born (The Baby; The Devil Within Her; It Lives Within Her; The Monster; Sharon's Baby) (1975) [Dr. Finch]: Decapitated with a shovel by the possessed baby in the garden.
Telefon (1977) [Nicolai Dalchimsky]: Poisoned when Charles Bronson forces him to swallow cyanide. (Thanks to Jake and Gordon)
Deep Cover (Blade on the Feather) (1980) [Professor Jason Cavendish]: Commits suicide (off-screen) by shooting himself, after Tom Conti encourages him to do so.
The Pumaman (L'Uomo puma) (1980) [Kobras]: Killed in an explosion when his helicopter crashes after a struggle with Walter George Alton.
Alone in the Dark (1982) [Dr. Leo Bain]: Hacked to death (off-screen) with an axe by Martin Landau; the scene cuts away as Martin confronts Donald. (Thanks to Johan)
Warrior of the Lost World (Il Giustiziere della terra perduta; Mad Rider) (1983) [Prossor]: Shot in the chest by Persis Khambatta after he orders her to shoot Harrison Muller; it's later revealed that the Donald who was killed was a clone/cyborg, and the real Donald is still alive.
Master of the Game (1984) [Salomon Van der Merwe]: Commits suicide by shooting himself. (Thanks to Gordon)
Phenomena (Creepers) (1985) [Professor John McGregor]: Stabbed to death by Daria Nicolodi on his staircase wheelchair-lift. (Thanks to Johan)
Treasure of the Amazon (Greed) (1985) [Klaus von Blantz]: Hanged by a hook through his tongue by Emilio Fernandez. (Thanks to Johan)
Specters (Spettri) (1987) [Professor Lasky]: Fatally injured (off-screen) by the monster in the catacombs; he dies shortly after John Pepper discovers him. (Thanks to Johan)
River of Death (1989) [Heinrich Spaatz]: Burned to death, along with Robert Vaughn, in a fire that breaks out after Cynthia Erland's gun goes off and hits some lab equipment. (Thanks to Johan)
Ten Little Indians (Death on Safari) (1989) [Justice Lawrence Wargrave]: Commits suicide by drinking poisoned wine. (Thanks to Johan)
Shadows and Fog (1992) [Doctor]: Killed (I can't recall the exact method) by Michael Kirby in Donald's office.
Halloween H20: 20 Years Later (1998) [Dr. Sam Loomis]: Revealed to have died (off-screen) of old age/natural causes in the years between sequels (since Donald died in real life before the making of this film). (Donald also appeared to die in several of the previous Halloween sequels, but always came back for the next one.)