Evidence 1: Everyone accuses Goodman when his car turns on by it self.
Evidence 2: Charlie shoots Pete Van Horn on the street.
Evidence 3: The aliens watch as the people of Maple street fight.
Theme: Mankind is it's own worst enemy
There are three pieces of evidence in "Monsters are Due on Maple Street" that support the theme man-kind is it's own worst enemy. The first piece of evidence is when Les Goodman is accused of being an alien because his car turns on by itself. This shows that nobody considered him being a victim as well just because it was his car that turned on and nobody had any other evidence. The second piece of evidence is when Pete Van Horn is shot on the street by Charlie when he isn't visible in the darkness. This shows that Charlie didn't even hesitate and that he was just ready to kill without considering anything. The last piece of evidence is when the aliens are watching the people of Maple Street fight. This shows that the Aliens conquered Earth by barely doing anything. Those are the three reasons why I think that the story's theme is "man-kind is it's own worst enemy".