It's a movie I saw as a kid, maybe 35 years ago where this guy accidentally hits and kills a pedestrian with his car (or maybe it was a man on a bicycle).

Then he gets an anonymous, ransom style, "I saw what you did", type note in the post scaring him to death that he's going to be caught. He tries to work out who sent him the note, guesses who it might be and kills them to protect himself, but after the murder, he gets a new note. This happens a couple of times until the end where you see him sitting at his desk in frustration, opening his drawer and finding cut up newspapers. Its then he realises that he was sending the notes to himself.

I'd love to see this again if anyone can tell me what it was.

Thanks so much in advance,

Ana