Jackson is driving at a couple key themes. One of the main ones is simply the danger of conformity. Look at how the villagers are perfectly willing to abandon their individual daily activities to unite for such a violent tradition/ceremony. Old Man Warner personifies this theme well. He comments how there has always been a lottery and there should always be one. He simply follows the tradition because it has always been in place. Mrs. Hutchinson did so too, until it was her turn to face the consequences of winning the lottery. When this happens, her entire family - and community - turns on her for the sake of tradition, and they all take part in killing her.
Another key theme is the human capacity for violence and cruelty. Again the community doesn't question such a violent tradition. They just obediently follow along. The fact that villagers can be so normal and docile one moment, and then turn on a fellow villager so cruelly a few minutes later is truly shocking. This theme is further highlighted by the excellent article by Shirley Jackson stating the fallout and repercussions of publishing her story in 1948. Click on the last link below for access to the article. The final paragraph of that article is just as shocking as Jackson's story itself, for it elaborates on what some of Jackson's readers really wanted to know about the lottery.