The gang of Old Sarum men led by Walter Cunningham Sr. who congregated at the jail was an old fashioned Southern lynch mob. Their intent was to forcibly remove Tom Robinson from his jail cell and take him to a "necktie party": a hanging. Cunningham and his cohorts had planned well: They had already called Sheriff Tate away into the deep woods on a "snipe hunt"--a bogus report that led the sheriff on a wild goose chase, leaving Tom undefended except by Atticus, who had previously been alerted that there might be trouble. Lynchings were a fact of life in the Deep South, and there were many instances of white men taking the law into their own hands--vigilante justice--by hanging or shooting black victims accused of some sort of crime or misbehavior, usually against whites. Cunningham was prepared: He came with plenty of men already emboldened by the aid of alcohol, and they were prepared to harm Atticus if necessary. But as Atticus pointed out later, "a mob's always made up of people," and Scout's innocent conversation with Mr. Cunningham reminded him that he was human and that maybe he wasn't capable of murder after all.
As was mentioned in the previous post, the Old Sarum bunch planned on removing Tom Robinson from his jail cell and lynching him. They wanted Tom Robinson dead because they did not want him testifying. The Old Sarum bunch were probably worried about the outcome of the trial and did not want to see a black man possibly win the case. Lynchings were prevalent in the Deep South during the Great Depression and occurred when racist white men decided to take matters into their own hands. Led by Walter Cunningham, the Old Sarum bunch arrives to find Atticus sitting outside of Tom's cell. They tell Atticus they've called Sheriff Tate out on a "snipe hunt" which means there is no one to help Atticus stop them. Fortunately, Atticus refuses to leave, and his children arrive out of nowhere. Scout successfully gets the attention of Walter Cunningham and he realizes the error in his judgment. Walter looks at the situation from Atticus' perspective and tells his men to leave. The Old Sarum bunch is not able to lynch Tom Robinson before his trial.