Why must they return the boy in the middle of the night?
In this entertaining little story, the kidnappers, Bill and the narrator, have taken the son of Ebenezer Dorset, who is an important person in Summit, Alabama, counting on his being willing to pay a good ransom for his only son. Over the next few days, the son, who wants to be called "Red Chief," shows himself to be more than a handful, making life hell for the kidnappers. He tries to scalp Bill with the steak knife, threatens to hit him with a rock, and finally knocks him out with a rock from a slingshot. While the narrator is gone, delivering the ransom note to Ebenezer, Bill, in desperation, tries to send the boy home, but he won't leave. This sets the stage for the next part of the story.
Ebenezer's response to the ransom note is to demand a payment of $250 to take the boy back. He writes that,
You had better come at night, for the neighbours believe he is lost, and I couldn't be responsible for what they would do to anybody they saw bringing him back.
Thus, we learn that Mr. Dorset is so relieved to be rid of his son that not only will he not pay a ransom to get him back, but also that he expects to be paid for taking him back. We also learn that the neighbors are clearly delighted that Red Chief has run away, so anyone bringing him back is in big trouble! Red Chief must be sneaked back to his home in the dead of night, so the kidnappers can be safe from the neighbors.