Jem has decided that what he, Scout and Dill did was wrong. What might account for Jem's change of heart?

Jem realizes just how close the children came to being hurt..... he also came to know what it felt like to lie to his father. Not only did the children lie about what they were doing, their excuse was a forbidden game. Jem had never been "whipped" by his father, and he didn't want to be whipped now. He didn't want to disappoint his father, and he certainly didn't want to shame him.