Lover of the Monster (1974) a.k.a Le Amanti Del Mostro

  The story of a scientist who moves to a new house with his wife to get a fresh start on their problematic marriage. It does not seem like a good start and things get even worse when he discovers some of his father's diaries that turn him into a crazed monster...
  Boring at times but certainly unique in most of its aspects, Lover of the Monster was directed by Sergio Garrone. It is in fact a twisted version of the classic novel about Dr. Jekyll and his dive into the abyss of hid mind. Here, the hero goes on a random killing spree while (rightfully) suspecting his wife of cheating on him. You just gotta love the mental atmosphere in these movies...
  Klaus Kinski plays the scientist that goes mad and Katia Christine plays the beautiful reason about that! Undoubtedly, this movie would suck even more badly if it wasn't for the almighty Klaus Kinski. Not only does he bring his ghastly mood with him in the film but also manages to freak you out with his eyes and movements. This man was either born a psycho or a really good actor!
Must watch this : with a glass of orange liqueur and some fine salty Italian cheese with the pleasant company of a lady friend. After the film, talk about relationships and cheating.
Trailer 
http://www.imdb.com/title/tt0421593/ 

Shit, you got the sheets all red again...

I've seen things, man...

Look, I know you are fucking my wife so drop it!