The Hanging Stranger

"The Hanging Stranger" is a science fiction short story by American writer Philip K. Dick, originally published in December 1953 in the magazine Science Fiction Adventures.
The protagonist, Ed Loyce, is a store owner who is disturbed when he sees a stranger hanging from a lamppost, but finds that other people consider the apparent lynching unremarkable.
He finds evidence that alien insects have taken over, manages to get out of town, talks to the police commissioner, who believes him, and after getting all the information about what Ed knows, explains that the body was hung to see if anyone reacted to it, anyone they didn't have control over. He then takes Ed outside and hangs him from a lamppost.