What is Shirley Jackson trying to say about human nature and about society?
Jackson's "The Lottery" is very much a scapegoat story.
It doesn't suggest that every society at all times kills people to appease someone or something--it shouldn't be taken literally (although that has happened in certain places at certain times--Nazis and others, civilians in Poland and the Ukraine, for instance, killed 11 million Jews, Gypsies, and gays during WWII).
The story suggests, though, that humans have scapegoating tendencies; that we tend to blame minorities, those weaker than us, those who can't defend themselves, for our problems and situations.
Notice particularly that the town, the people, their daily existences, are completely normal. The lottery is even taken so matter-of-factly that the subject of lunch--after the lottery--comes up.
Jackson is suggesting that given the right circumstances, even "normal" human beings are capable of atrocities, but again, that scapegoating is also an everyday occurence, though on a less tragic level.
In my opinion, what she is trying to say is that we are all brutal and that we all have this ability to check our morality at the door and go ahead and do what everyone else is doing. She is saying that this makes it so evil can happen even in a place that looks nice and peaceful.
The people in this village seem so nice. But then they go out and they are willing to kill one of their neighbors for no reason. The only reason they do it is because it is tradition -- everyone does it and they have always done it.