This is part of the My Twisted Valentine season at the Barbican. Full details here.
Chicago Reader review:
Leonard Kastle, a composer who turned filmmaker for this single feature (1970), brings a spare dignity and genuine depth of characterization to his exploitation subject—the series of murders committed by Ray Fernandez and Martha Beck in the late 40s. Fernandez (Tony LoBianco) is a third-rate gigolo who seems deeply in love with the dominating Beck (Shirley Stoler, the concentration camp guard of Seven Beauties); together they seduce and kill elderly, lonely women.
Here (and above) is an extract.