Satans of the Night (1972)

Zisis, a worthless layabout and father of two, has been toying with a hideous idea to get rich without breaking a sweat for quite some time now. And after careful planning, the ruthless opportunist summons up the courage to break into the home of his wealthy mother-in-law with his good-for-nothing friends. However, there's no such thing as a foolproof plan. As the poorly executed heist backfires on the ambitious robbers, tragedy strikes--a shocking act of unspeakable violence that will live in infamy.