What is particularly chilling about the ending of the story is that those individuals who had been particularly friendly with Tessie are some of the first to stone her. Mrs. Delacroix is a case in point, as at the beginning of the story we see that she is a friend of Tessie's through the way in which they casually exchange small talk. Yet the moment that Tessie selects the ballot paper bearing the black dot, those years of friendship are swept away as Mrs. Delacroix picks up a stone to kill her friend with:
Mrs. Delacroix selected a stone so large she had to pick it up with both hands...
We can just imagine her staggering along towards Tessie to crush her with this stone in her bloodlust. Jackson included this detail to reinforce her central point, which is the way in which logic and loyalty can be overwhelmed by the power of tradition and doing things because we have always done them that way. Tradition is shown to be a cancerous force in this brilliant short story that eats away at even the most sacred of bonds. Mrs. Delacroix's stone is one way of showing this, but note also how even little Davy Hutchinson is given pebbles to throw at his mother. All participate in this ritual murder, and all are shown to let logic and loyalty slide.
When Mrs. Delacroix struggles with a massive stone with which to crush Tessie Hutchinson, Jackson conveys the propensity and desire for violence in human nature.
When it was first published in the New Yorker, there were some readers of Jackson's story who wanted to know where such lotteries were held, and whether they could go and watch [http://www.newyorker.com/books/page-turner/the-lottery-letters].
These readers underscore Jackson's point made with Mrs. Delacroix's lugging of a huge rock: There is a deeply innate readiness and enjoyment of violence. In fact, in some people this inclination is so deep that it overrides other feelings.
That this propensity for violence is deeply rooted is suggested in the actions of Bobby Martin, who "had already stuffed his pockets full of stones" and the other boys who follow his example. After his mother has called him four or five times, she grabs his hand, but Bobby "ran laughing, back to the pile of stones." The act of stoning is almost like a sports event. Everyone falls into line, eager to begin. When Tessie Hutchinson objects, Mrs. Delacroix tells her, "Be a good sport, Tessie....All of us took the same chance." Even the children participate.