Scout is about to kill the bug when Jem stops her. It's clear that first off, the bug is afraid of Scout but it's only after he gets more comfortable and unrolls himself that she goes to strike. This mirrors the town's (especially Mayella Ewell's) treatment of Tom Robinson because he had finally gotten comfortable with someone in the town enough to not be on his guard all of the time and that's when she struck. The town barely even gives him any thought-they do not think he is worth saving, as Scout does not think that the bug is anything worth actually thinking about or saving.