In the story "The Monsters are Due On Maple Street", one of the good examples of mankind being it's own worst enemy is the time when people started finding scapegoat due to false accusations. It all started when people were blaming Les Goodman, then from him to Steve (thanks to Charlie), but it backfired to Charlie after he shot Pete Van Horn (as shown in the picture), and lastly it came on Tommy for "knowing to much" about what was going on.
This picture shows that even Steve's reasoning couldn't stop the residents from turning against each other into a mob. As soon as the lights went flickering on and off, house to house, people started to go crazy thinking that the extraterrestrial creature(s) were messing with them in some sort of way. So they started a riot that almost tore up maple street. Another example of humans destroying their own selves in this story.
Only in the end is it revealed to us that aliens were messing with the Residents of Maple Street. It turns out that all they have to do is turn off all the electricity in the place, toy around with some of the lights , and watch as mankind started turning against each other through fear and suspicion. This proves that man is its own worst enemy due to the fact that they can hurt each other in different ways; even just by falsely accusing each other of different things, such as (in this case) blaming one aother for being an "alien".