The incident concerning Scout, Jem, and the roly-poly occurs just after Tom Robinson is killed, and Atticus, Calpurnia, Dill, and Jem go to inform his widow that he has died. Robinson's death has a great impact on Jem; he is at an age where he understands the waste and injustice that his killing signifies. When Scout, who has been playing with a roly-poly, tires of her game and makes a motion to kill it, Jem tells her not to, and to "set him out on the back steps" instead. When Scout asks why she couldn't "mash" the bug, Jem responds, "because they don't bother you." Scout, being less mature than her brother and unaware as yet about Tom Robinson's death, replies sarcastically, "Reckon you're at the stage now where you don't kill flies and mosquitos now," and declares that if a redbug were to torment her, she would not hesitate to destroy it. Scout does not understand what her brother does about the tremendous tragedy of hurting a living creature who never did anyone any harm.
Jem's act of telling Scout not to kill the roly-poly ties directly to the central theme in the book. As indicated by the title, "to kill a mockingbird," an innocent creature who does no harm, is wrong. Tom Robinson's murder is the greatest atrocity because he is a human being who, like the mockingbird and roly-poly, is killed even though he has never hurt anyone. In preventing Scout from thoughtlessly snuffing out the life of the roly-poly, Jem is demonstrating that he understands the underlying tragedy of Robinson's death (Chapter 25).