When Jem tells Scout to leave the roly-poly alone and not kill it, she thinks he has "gone soft" and become silly like a girl about things.
In Chapter 25 of To Kill a Mockingbird, Harper Lee utilizes this seemingly insignificant incident of Scout's thoughtless toying with a species of woodlice known as a roly-polies, or doodle bugs, in order to expose Jem's growing sense of the world outside himself. While Scout only thinks of her own enjoyment in squashing this oval-shaped crustacean that feeds on decaying matter, Jem perceives her act as one of gratuitous cruelty that deprives this little creature of its life.
Clearly, Jem's experience of being in the courtroom and witnessing the unjust conviction of Tom Robinson, and later his death, has a profound impact upon him. Having become sensitized to cruelty, he now wishes none done to any living creature. Interestingly, this episode with Scout and Jem precedes the report by Scout of Mr. Underwood's editorial, in which he decries the senseless death of Tom Robinson that was as thoughtlessly cruel as the crushing of some lower form of life:
He likened Tom's death to the senseless slaughter of songbirds by hunters and children.
When Scout ponders what Mr. Underwood has written, she realizes that Tom was convicted, not by the law, but in the "secret courts of men's minds." This unfair death of Robinson is not unlike, although on a much smaller scale, Scout's intention of crushing the roly-poly.