Well the movie starts with Gianni Garko’s character witnessing the gangrape of his own wife (I think) and after that he becomes mentally unstable I think you could call it and causes the dead of a lot of people 
The basic giallo plot but here of course we know who is doing the killings.
It’s on the first of Koch Media’s Enzyklopädie box sets but otherwise someone has uploaded the whole thing on youtube:
Check the music playing from about 4:45 onwards. Pure giallo.