The Devils of Monza(1987)

The young noble Don Monza is caught and thrown out by the nun Virginia de Leva when she catches him flirting with one of her nun sisters across the stone wall between their gardens. He hears the rumor that she'd have an affair with her trustee - so he kills him in a set-up duel and aims to take his place. Against her initial resistance, Monza manages to win her heart - and to get into her bed. This doesn't stay a secret for long, but Monza, now in love, does everything to keep it going.