The Honeymoon Killers (1970)

Martha Beck an obese nurse who is desperately lonely joins a correspondence club and finds a romantic pen pal in Ray Fernandez Martha falls hard for Ray and is intent on sticking with him even when she discovers hes a con man who seduces lonely single women kills them and then takes their money She poses as Rays sister and joins Ray on a wild killing spree fueled by her lingering concern that Ray will leave her for one of his marks