Why push the boy?

Ella pushed a boy off of some play equipment at her school. When her mom asked why, she said, "Clara told me to, to get back at the people who killed her."

The thing is, we learned later that that boy did not kill Clara. It was the strange man next door who pushed her off of the treehouse ladder.

So, why would Clara tell Ella to push the boy if he had nothing to do with it?