Why did the cunninghams back away from Atticus when Scout interupted at the jail cell?

they weren't about to beat oder kill him in front of his daughter. seeing her brought them back to their senses and possibly made them ashamed of what they were about to do so they stopped and left.

Scout reminded their leader, Mr. Cunningham, that he has a family and he's not a bad guy. That's why he told the rest of the group to leave.

Shame.