Recalling the mob scene of Chapter XXII of The Adventures of Huckleberry Finn, in which Sheburne confronts a mob--
The pitifulest thing out is a mob; that's what an army is--a mob; they don't fight with courage that's born in them, but with courage that's borrowed from their mass and from their officers. But a mob without any man at the head of it, is beneath pitifulness--
In To Kill a Mockingbird, the lynch-party are the "Old Sarum" bunch. "an enormous and confusing tribe" from the northern part of the county. Like the mob in Twain's novel, they, too, are lacking in courage individually, but they possess it as a group. However, it is little Scout who turns it into "the pitifulest thing"; for, after she and Jem follow Atticus to the jailhouse, they see the old dusty cars pulling into the lot by the door where Atticus sits.
When one man puts his hands on Jem, Scout kicks him. As the tension mounts, and a man says, "All right, Mr. Finch, get 'em outa here," Scout recognizes Mr. Cunningham and addresses him, returning him to the office of leader of small mind. After they lose their leader, as suggested by Sheburne, the mob becomes the "pitifulest thing" so Mr. Cunningham tells them to follow him home.