Evidence 1: The scene in which all the residents accuse Les of being the "monster" when his light porch turned on.
He always was an odd ball.
Evidence 2: Charlie shoots Peter Van Horn thinking he was the "monster¨.
Evidence 3: After Charlie's light turned on they all blamed him and soon the blame game began.
IT WAS HER!
IT WAS YOU!
The theme of "The Monsters Are Due on Maple Street" is don't let fear control your actions. I believe this is the theme for multiple reasons. One reason being that after Les Goodman's car started on its own they all started to accuse him and distrust him. This fits to the theme because they let their fear of Les Goodman control them and distrust him. My second reason is Charlie shot Peter Van Horn out of fear. He let his fears control him in that moment and he killed him. My third and final reason is they all turn against each other because of fear. Which proves that the theme i choose don't let far control your actions is true.