What theories exist as to what would prompted Shirley Jackson to write such cruel story? I need two reasons...This is for my English class and I...

You should read the "themes" section at the link below.
There you will find some theories that have been advanced as to the purpose of this
short story.

Shirley Jackson published this story first in
The New Yorker magazine. Both she and the magazine received a lot
of hate mail about the allegedly cruel story and many people canceled their
subscriptions. In reaction to this, Jackson herself explained that she was trying to
show man's inhumanity to man in the story, to focus on the inherent cruelty within the
human soul. She reports that even her parents didn't like the story and urged her to
write something nice.

The story underlines the danger of
rigid thinking, perhaps even prejudices, and doing something just because it has always
been done in such a way, a sort of human lemming theme. Lemmings are rodents who people
incorrectly believe commit mass suicide when they migrate. The idea is that lemmings
follow each other to their deaths just because that is what they do. In this short
story, a few people question the purpose of the lottery, and yet no one has the guts to
stand up and say, "This is evil!" This theme can apply to many negative aspects of human
society, don't you think?