Ace of Spades Movie Club: The Heartbreak Kid, original version, 1972, with Charles Grodin and Cybil Sheppard
This movie is currently unavailable to buy legally, because the company that bought it went out of business or something. It's in license hell. But this copy is on YouTube. I've never seen it, but I'm watching it now. I understand that it's much darker and the main character is much crueler than in the Farelly Brothers 2007 version, where they justified his actions by making his new bride crazy and a serial liar. Who has a massive coke debt and also has no job. Says so in the dictionary.
Update: It's pretty good, but it is a lot darker and more uncomfortable. The Farelly Brothers movie takes the basic script and turns every scene into a big comedy set-piece. I think Ben Stiller's endless lies were funnier than Charles Grodin's. Interestingly, Charles Grodin does the Fake Federal Agent act here that he later repeats in Midnight Run. He does the same act with different words.
BTW here's the original short story the film is based on. Now I want to watch the 2007 remake again.