One thing that Jackson is saying about people in general is that behavior that is not examined in light "clear and sunny, with the fresh warmth of a full-summer day," is dehumanizing of self as well as potentially destructive of others' lives.
I agree that the primary focus of this chilling tale is the deadly nature of our mindless adherence to tradition. A reflective reader is forced to think about any personal or societal rituals we adhere to without thinking. While there is something to be said for tradition and routine, it is the unthinking, unquestioning adherence to such things which deserves some consideration. When little Davey is encouraged to help kill his mother and when Tessie is willing and even eager to sacrifice her own daughter to save herself, we have to examine our own behaviors.
That Tessie Hutchinson is the only objector to the lottery, and that she is given no credence is very disturbing. This sheep-like mentality of people is a concept of which Jackson makes the reader aware, extending to how dangerous it is to be the "open-mouthed consumer" (to use a term of psychologist Dr. Erich Fromm) of such things as media ideas. Jackson's story also broaches the identification of the innate delight in violence that is mankind's.
This modern gothic tale really forces us as a society to take a long hard look at ourselves and consider what kinds of behaviour that are completely unjust, cruel and barbaric we are perpetuating, condoning and participating in. You only need to think of some activities in other cultures like female circumcision or the caste system in India to see tradition in all its terror at work. The problem is though that it is so much easier to spot these things in others than ourselves.
Ah, you are looking for the universal element of this haunting story by Shirley Jackson. Through the lottery, Jackson shows us the natural acceptance of routine and ritual that pervade our lives. Because a tradition has always been carried out, we accept it without question, just as the citizens of Jackson's small town accept the tradition of the lottery. They do not question its morality because it has always been a part of their lives.
Perhaps traditions regarding segregation in our country could be compared to the lottery. Before the Civil Rights movement of the 1960s, many white people did not question the morality of having separate black and white schools, separate restrooms, restaurants that would serve only whites. This was the way things had always been, so this type of discrimination was so ingrained into our psyches that we did not question the unfairness of this situation.
There are probably many practices today that we accept that later others will see as barbaric and savage. Jackson asks us to examine more closely and more critically our own customs, practices, and routines.
I think the author was trying to say that we get stuck in tradition. Just because we did something a certain way once, we keep doing it especially if it worked then. As times change, we need to consider other ways and think outside the box. We don't just have to go along with the circumstances of a situation, we have the abilities to think and reason and come up with different approaches to problems.
"The Lottery" demonstrated the ill of a society which refuses to change. There may be different reasons for their refusal ranging from ignorance to determined sticktuitiveness.
To label this story with any one theme I think it would be a caution about tradition.