Twitch of the Death Nerve
Our Movie of the Week went under many different titles when released in 1971, including Bay of Blood, Carnage, Chain Reaction and Last House on the Left Part 2. Director Mario Bava broke tradition, both of with his own oeuvre and with film in general, by showing a string of gruesome murders with little buildup of suspense, making for a movie that disturbs in its seeming indifference to the well-being of its characters. Hardly anything like this had been done before, and wouldn't be again until, maybe, the release of Friday the 13th many years later. In fact, many of the set pieces in 'Friday' are lifted directly from this film. Not on DVD, so see it here while it lasts.
Here's a gorgeous trailer for its original American release, done entirely in duotone and solarized film: