When discussing rituals and human nature as shown in Jackson's "The Lottery," you must include the concept of blind, or mindless, ritual and its relationship to human nature. You might even discuss whether any ritual can be mindful or whether all must be--by definition--mindless and blind and unthinking. Personally, I think there can be rituals that are not blind and mindless; your research will guide you in forming a reasoned opinion.
I agree with mwestwood. Whatever your thesis is it needs to be something that can be argued. We can't really argue that mindless following of ritual can be harmful--that's a common sense issue. What could be argued is something like "The mindless following of ritual in Shirley Jackson's "The Lottery" is shocking, yes, but no more so than the mindless rituals observed by modern society," or "The use of a scapegoat in Shirley Jackson's "The Lottery" is appalling, but this is still a trend followed in our more modern society."
You may also wish to research the concept of the scapegoat and how it has become entrenched in even modern society, at least figuratively. In the Iran-Contra scandal of Ronald Reagan's administration, for example, Oliver North became the scapegoat. There is an inherent predilection in humanity for sadistic behavior, and it manifests itself in choosing a scapegoat so that the others can feel relieved of the problem, or better about themselves.
Your thesis statement largely depends on what you intend to say about rituals and human behavior, what position you intend to take. I'm assuming, since the question is asked in "The Lottery" discussion site, you're taking the position that rituals are both integral to (inherent in?) human behavior and also perhaps responsible for some bad human behavior. In that case, your thesis might read something like this: "Human behavior is controlled, at least to some degree, by its inherent reliance on rituals, as demonstrated in Shirley Jackson's "The Lottery." Perhaps your position is that rituals lead to bad behavior, or that mindless adherence to empty rituals is a cause of human suffering.
Whatever your position is, the thesis must reflect that. Your research should help you determine your position; if you haven't done enough of that yet, this may be quite difficult for you. In any case, unless I must stick with a predetermined thesis, I always see a thesis as a work in progress until I'm actually working on the paper. What I think I'm going to find isn't always what I actually find; and what I think I believe may not end up being what I truly believe. That's the beauty of research.