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I remember watching the end of
In Cold Blood on TV late one night when I was a teenager. It was so shocking. I described the movie to my dad the following morning: "two guys with shotguns kill this family". He knew what I was talking about in a second.
And the fact that it stars Robert Blake makes it even creepier now.
(The true story, upon which the movie and book are based, is described
here in Wikipedia).