Why was it a risk to put a Cunningham on the jury? Why did Atticus do it anyway?

The Cunninghams had a personal grudge against Atticus... they threatened both Atticus and Tom when they arrived at the jail with the lynch mob. After this incident, however, Atticus knew that Cunningham had respected the way he stood his ground... he also believed that he was a fiar man.

He said the Cunninghams hadn’t taken anything from or off of anybody since they migrated to the New World. He said the other thing about them was, once you earned their respect they were for you tooth and nail. Atticus said he had a feeling, nothing more than a suspicion, that they left the jail that night with considerable respect for the Finches. (Ch. 23)