What is the significance of Jem's reaction to Scout when she starts to kill the roly-poly?

The roly-poly incident is an example of Jem's increasing maturity. Having witnessed Tom's trial and his family's reaction of his death, Jem has an even greater sense of the need to protect the innocent. Therefore, the roly-poly bug is a symbol of the weak and oppressed who are often "stomped on" by society. Jem believes in the equality of all people, and his choice to protect the roly-poly demonstrates how deeply ingrained this value is. Jem is becoming a young man of honor and moral virtue, just like his father.