In "All Summer in a Day," why did William lock Margot in the closet?

William locks Margot in the closet because the sun is coming out and he does not want her to see it.

The problem between Margot and the other kids is that she is from Earth and they have lived on Venus for all of their lives.  They are jealous of Margot because she has seen the sun, and they do not remember the last time it came out when they were toddlers.  Margot also does not relate well to any of the kids from Venus.

The appearance of the sun brings out the worst in the kids. They are very excited because the sun only comes out once every seven years.

They turned on themselves, like a feverish wheel, all tumbling spokes. Margot stood alone. She was a very frail girl who looked as if she had been lost in the rain for years…

Margot has been on Venus for only five years. She remembers the sun, but the children cannot stand the idea.  They have turned to bullying to deal with the situation, because Margot has not assimilated into Venus life. She does not play their games and has not made friends.

"All a joke!" said the boy, and seized her roughly. "Hey, everyone, let’s put her in a closet before the teacher comes !"

The children do not really think about what they are doing when they put Margot in the closet.  They make it a “joke” even though it is supremely cruel.  When the sun comes out, the children simply forget about her in all of the excitement.  They seem to have intended to pretend to lock her in there so she would miss the sun, but then they actually did it.

This story is a good example of the pointless cruelty of bullying.  If someone is different, then that person becomes a target.  This is even more true with children, who are intolerant of uniqueness and privilege.