In the short story "The Lottery," I don't seem to understand why they stoned Mrs. Hutchinson to death. What did she do wrong?

In this short story Mrs. Hutchinson was stoned to death by the villagers as a custom of sacrifice.  Every year the whole village got together, reached in a black box and drew out a piece of paper.  If the paper had a black mark on it that person would be sacrificed for the "good of the village." 

Mrs. Hutchinson didn't do anything wrong, she was just unlucky enough to draw the paper with the black mark.