When las goodman says , "you're letting something begin here that's a nightmare " , what does that mean?

Les Goodman says, "You're letting something begin here that's a nightmare" when the neighbors start to suspect each other of being aliens. He sees how every person's oddity is being called into question. The people start to act like a mob and take on the mob mentality which can be dangerous. When people act as a mob, they have strength in numbers and are quick to believe things or support things they wouldn't normally do if they were alone or with a smaller group. In this case, the people are quick to back each other up by bending the truth and making false accusations because they are scared and looking for something to explain what is happening on Maple Street.