Scout finds a roly poly on the back porch one night and right before she smashes it, Jem stops her. What is his reasoning? Why is he suddenly concerned about a killing bugs?

Jem has finally internalized the lessons Atticus was trying to teach them about the sin it is to harm defenceless creatures like mockingbirds. Scout asks why she cannot smash the bug, Jem replies, "Because they don’t bother you" (Lee, 242 my copy). Scout will later internalize this for herself when she sees people like Tom Robinson and Boo Radley as mockingbird-figures.