|On Her Majesty's Secret Service (1969) [Contessa Teresa 'Tracy' Di Vincenzo Bond]: Shot in the head when Ilse Steppat machine-guns her car while driving past with Telly Savalas; her body is shown afterwards when George Lazenby gets back in the car and discovers that she's been hit. (Thanks to Claire)
Julius Caesar (1970) [Portia]: Commits suicide (presumably off-screen) by swallowing hot coals. (I haven't seen this version, but I'm familiar with the play.)
Theatre of Blood (Much Ado About Murder) (1973) [Edwina Lionheart]: Hit on the head with an awards statuette by a homeless woman during a struggle in the burning theatre; she dies in the arms of her father (Vincent Price), who then carries her body up to the roof before falling to his death. (Thanks to Anastasia)
Oresteia (The Serpent Son) (1979) [Clytemnestra]: Killed (I don't know the exact method, but I assume she was stabbed to death) by her son (Anton Lesser), in revenge for her murder of her husband (Denis Quilley). (I haven't seen this mini-series, but I'm familiar with the history/myth.)
Hedda Gabler (1980) [Hedda Gabler]: Commits suicide by shooting herself in the head. (I haven't seen this version, but I'm familiar with the story.)
Evil Under the Sun (1982) [Arlena Stuart Marshall]: Strangled by Nicholas Clay in a grotto; her body is shown lying on the beach when Peter Ustinov examines it, and the murder is shown in a flashback later on. (In the scene before Peter's examination of the body, the "body" lying on the beach is not Diana, but Jane Birkin playing dead with her face covered, as part of their scheme to create an alibi.) (See also Louise Delamere in the 1991 version.)
King Lear (1984) [Regan]: Poisoned by her sister (Dorothy Tutin). She is still alive (albeit very ill) when we last see her; her off-screen death is revealed when a messenger brings the news. Her body is not shown.
Bleak House (1985) [Lady Honoria Dedlock]: Dies of exposure/pneumonia in a cemetery.
Cannon Movie Tales: Snow White (Snow White and the Seven Dwarfs) (1988) [Evil Queen]: Rapidly ages to death and crumbles to dust after she breaks the magic mirror. (Thanks to Drew)
Rebecca (1997) [Mrs. Danvers]: Dies after Charles Dance falls down a flight of stairs while trying to carry her out of the burning building; it's unclear whether it was the fall or smoke inhalation that killed her. (Thanks to Brian)