Shock (1977)

Also known as Beyond the Door 2, Shock is -according to Wikipedia- Mario Bava's final film. He died of a heart attack in 1980. The film has plots, murder, insanity and a possessed child. What more can you ask of a horror film? I like this type of film much better than gore-filled slashers.

trailer:

1000 Misspent Hours says it "displays a hell of a lot more imagination than most of its contemporaries, and gets tremendous mileage out of many surprisingly simple tricks" but doesn't advise it as an introduction to Bava. ClassicHorror.com says,

Is it a psychological horror film about the maddening effects of guilt, or is it a supernatural thriller filled with shocks and otherworldly ghouls? Unfortunately, since it can't make up its mind, Shock is neither.

Images Journal says,

Shock pales in comparison to Bava's best movies, but it's an effective thriller with an oppressive atmosphere that reeks of decay and death. It may well be his best movie of the '70s.