A topic sentence that may help you expand more upon the lynch mob incident would be "racism is like a disease that blinds people to any common sense, and brings the inhumanity out in those who embrace it".
The reason why that sentence may work is because you can write largely about how Mr. Walter Cunningham's initial desire to lynch Tom Robinson was completely overturned with just a small intervention of words by Scout, who was around eight years old at the time.
"... it took an eight-year-old child to bring 'em to their senses..."
You could also expand upon whether Walter Cunningham was there because he wanted to lynch Tom for being a potential criminal accused of raping a woman, or if he was there wanting to lynch Tom for being black. Regardless of the source of Cunningham's motivation, Scout's words to Cunningham immediately bring him to his real "here and now." He is a father, a hard-working farmer, a neighbor of the county, and an overall decent person. Those could have been factors that led him to realize that he had no business being part of a lynch mob.
But that is only Walter Cunningham. Like a disease, racism and prejudice divide the Maycomb community, creating dissension and discrimination in every aspect of life, even church. People like the Ewells and many others may have no qualms about lynching a man just because he is black. We even witness the dangerous combination of racism, ignorance, and violence that is embedded in the crazy mind of Bob Ewell. Therefore, racism brings out the inhuman, cruel, and weakest traits out of people.