Deep Inside (1968)

Millicent Redmond, a powerful, frigid woman, is the informal leader of a group of women who live near a beach during one summer. Millicent delights in manipulating her friends by exploiting their weaknesses. She maneuvers Lina, a nymphomaniac, into making desperate attempts to find satisfaction; toys with Neva and Jean, her lesbian roommates from college days: caters to Mavis, an older woman who craves young lovers; and helps to draw Pam Lewis into an affair with a psychotic artist. Millicent maintains control until her games lead to murder. Finally, her friends see Millicent's evil motives and turn on her.