Not calling Marathon Man a Horror though doesn't dilute it's effect. It should be considered one of the scariest films precisely because of how real he is.
Damn it! Thought I had you there.
I know strictly speaking Marathon Man might not qualify. But if you need monster to qualify as horror, this film has a human monster.
Henry : Portrait of a Serial Killer isn't an horror movie and this is the scariest film I've ever seen. Reality is way more scarier than vampire, zombie, ghost and other fictional monsters... at least for a grown-up