|Scotland Yard Inspector (Lady in the Fog) (1952) [Danny McMara, Hit-and-Run Victim]: Hit by a car. (I haven't seen this, but the IMDB's identification of his character gives away his fate.)
Hamlet (1970) [Claudius]: Stabbed and poisoned by Richard Chamberlain. (I haven't seen this verison, but I'm familiar with the play.)
Julius Caesar (1970) [Cassius]: Stabbed to death by Damien Thomas, on Richard's own orders. (I haven't seen this version, but again, I'm familiar with the play.)
Antony and Cleopatra (1974) [Marc Antony]: Commits suicide by throwing himself onto his sword; he dies shortly afterwards while talking to Janet Suzman.
The Night Child (Il Medaglione insanguinato; The Cursed Medallion) (1975) [Michael Williams]: Stabbed in the stomach by the possessed Nicoletta Elmi when he hugs her. (Thanks to Andrea)
Screamers (L'Isola degli uomini pesce; Island of the Fishmen; Something Waits in the Dark) (1979) [Edmond Rackham]: Torn apart by the fish-creatures when he falls into the water at the end of a struggle with Claudio Cassinelli. (Thanks to Johan)
Zombie (Zombi 2; Island of the Flesh-Eaters; Island of the Living Dead) (1979) [Dr. David Menard]: I vaguely recall that he was killed in this movie, though I can't remember the details.