Orson Scott Card
A great rabbi stands teaching in the market place. It happens that a husband finds proof of his wife's adultery, and a mob carries her to the market place to stone her to death. The rabbi walks forward and stands besides the woman. Out of respect for him the mob forbears, and waits with stones heavy in their hands. "Which of you is without sin? Let him cast the first stone."
The people are abashed, and they forget their unity of purpose in the memory of their own individual sins. Someday, they think, I may be like this woman, and I'll hope forgiveness and another chance. I should treat her the way I wish to be treated.
As they open their hands and let the stones fall to the ground,the rabbi picks up one of the fallen stones, lifts it high over the woman's head, and throws it straight down with all of his might it crushes her skull and dashes her brains onto the cobblestones.
"Nor am I without sin," he says to the people. "But if we allow only perfect people to enforce the law, the law will soon be dead, and our city with it."
from his book Speaker For The Dead.