Final answer:
The author chooses to have The Misfit shoot the grandmother last in order to show her change through her dialogue and give her a chance to teach The Misfit about the South.
Step-by-step explanation:
The author chooses to have The Misfit shoot the grandmother last primarily to let readers see her change through her dialogue with The Misfit. This choice allows the author to showcase the grandmother's transformation and her attempt at convincing The Misfit to turn himself in to the police. By saving the grandmother for last, the author highlights her final chance to make an impact on The Misfit and potentially teach him about the South and its values.