Das verräterische Herz (1979)

A man is sitting in a padded cell. He remembers how he got there. Munich in the run-up to Christmas: The loner roams the city, has abnormal ideas when looking at the most harmless things and wants to turn his perverse fantasies into reality. To do this, he chooses an old man whom he happens to see in a shop. He perfectly plans a perfidious, cruel murder.