Fear makes people lash out against what scares them in order to return to their comfort zone. They do this even if they have to destroy innocent lives along the way. What does Bob Ewell fear?

I think Bob Ewell fears himself. He hates what he has become and fears what is going to become of him. Bob Ewell retreats into hate and sociopathic acts to hide behind a world that he fears has left him behind to rot beside a garbage dump.