|Room at the Top (1959) [Alice Aisgill]: Killed in a car crash (implied to be suicidal).
The Sleeping Car Murder (Compartiment tueurs) (1965) [Eliane Darres]: Killed by Claude Mann (though I don't know the details).
The Deadly Affair (1966) [Elsa Fennan]: Killed when Maximilian Schell applies pressure to her carotid artery while sitting next to her in a crowded theatre; it isn't apparent that she's being murdered until afterwards, when she falls out of her seat as the other audience members leave.
Army of Shadows (L'Armee des ombres; The Shadow Army) (1969) [Mathilde]: Shot to death by Lino Ventura in the street, after she had betrayed him to the Gestapo. (Thanks to Zoran)