|Quo Vadis (1951) [Young Christian Girl]: Eaten by lions (off-camera), along with Nora Swinburne, in the arena; we only see the spectators' reactions as the lions attack. (Thanks to Gerardo)
The Three Musketeers: Milady's Vengeance (1954) [Milady de Winter]: Presumably executed by beheading. (Thanks to Gordon)
The Hellfire Club (1960) [Isobel]: Shot by her husband. (Thanks to Anonymous)
A Study in Terror (Fog) (1965) [Angela Osborne]: Burned to death (along with John Fraser and Peter Carsten) after a fire breaks out during a fight between John Fraser and John Neville. We see Adrienne and Peter Carsten through the flames as they fall over a railing.
The Idiot (1966) [Nastasia]: Killed by Anthony Bate. (I haven't seen this version, but I'm roughly familiar with the basics of Dostoevsky's story.)
The Viking Queen (1967) [Beatrice]: Stabbed in the stomach with a spear by a Roman soldier while riding into battle on her chariot. (Thanks to Bill)
Moon Zero Two (1969) [Elizabeth Murphy]: Shot in the stomach with a ray-gun by Warren Mitchell. (Thanks to Bill)
A Clockwork Orange (1971) [Mrs. Alexander]: Dies of pneumonia off-screen, during the passage of time between scenes. We only see Patrick Magee informing Malcolm McDowell about her death after the fact.
Vampire Circus (1972) [Gypsy Woman]: Bitten on the throat by Anthony Higgins in order to use her blood to bring the head vampire (Robert Tayman) back to life.