Who is The True Believer from Barabbas and what is their importance?

The True Believer is standing at the stoning pit when the Hare-Lipped Girl is about to be stoned. The girl's accuser, a blind man, is to cast the first stone. He misses her repeatedly until the True Believer throws his stone and strikes the girl. Barabbas, who is watching, then quietly walks over the man and kills him with a knife.