Evidence 1: The residents on Maple street assume that Les Goodman and his family of being the aliens that came over head because his car started by itself.
Evidence 2: The residents of Maple Street use Mrs. Brands complaints about the radio set Steve has been working on to their advantage and Charlie says that he is talking to someone on the radio.
Evidence 3: The aliens state that if you just turn off a few lights and turn on cars that humans will start to turn on each other so there is no need to use bombs.
The theme of "Monsters on Maple Street" is Humans are their own worst enemy. I believe this because the people on Maple Street start to accuse and are very quick to judge their neighbors as shown in evidence 1. They start to judge something they don't understand like when Les Goodman's car started and stopped randomly when he was away from the car. Also as shown in evidence 2 Mrs. Brand was complaining about her husband working on a radio all night, like a normal wife would but they use this to their advantage to accuse Steve just because of that complaint. Last but certainly not least the aliens that did come said that if you turn off a couple of lights and turn on cars that humans themselves would turn on each other out of judging and confusion in other words fear. This furthers my statement that indeed the theme of "Monsters due on Maple street is Humans are their own worst enemy.