I think the important word here is "think." Atticus is suggesting that the men think about what they are about to do before they go through with their murderous intentions. As Atticus tells Jem in the subsequent chapter,
"A mob's always made up of people... Mr. Cunningham was part of a mob last night, but he was still a man." (Chapter 16)
Atticus is appealing to them as a friend and as a man, and he hopes that they will stop and think individually about their actions, rather than as a group--"a gang of wild animals." Atticus's "dangerous question"--"Do you really think so?"--is one he has used before with his children, and it is always meant to question the wisdom of an action, such as when Scout is about to make a rash move while playing checkers, and
"Bam, bam, bam, and the checkerboard was swept clean of my men." (Chapter 15)
Or when Jem misunderstands something, and Atticus gives him a reference book in order to find the answer himself. Atticus hopes that Cunningham and his friends will stop and think about the seriousness of their plans before making a decision they cannot reverse.