Answer:
Jem chose not to return the blanket so as to save Boo Radley from getting into trouble with his father. He told Atticus "<em>I swear to God he ain’t ever harmed us, he ain’t ever hurt us, he coulda cut my throat from ear to ear that night but he tried to mend my pants instead… he ain’t ever hurt us, Atticus"</em>
Explanation:
In Harper Lee's novel "To Kill A Mockingbird", the residents of Maycomb seem to all have a common understanding not to get too involved with the Radleys. They, including Atticus Finch, also tell their children to not disturb them. That was how the Radleys came to be regarded as something to be abhorred.
But Chapter 8 shows the realization of not only Jem but Atticus himself that Boo Radley was far from who they had been thinking he'd be. One such evidence in this chapter is the fire scene where all of Maycomb had come out to help put out the fire from Miss Maudie's place. Atticus had asked them to stay put at the Radley's gate where they stayed till everything was over. So engrossed were they with the scene that they did not even realize Boo, who was a recluse, had actually ventured out of the house to put a blanket over Scout who must have been feeling cold. This shows how sensitive Boo is, not forgetting he was the one who had left those numerous 'gifts' at the tree hole and had also mended Jem's torn pants before. In his decision to not return the blanket to him, Jem's action signifies he doesn't want Boo to be scolded or even have any hard time with his father for his kindness to Scout. This incident made Jem realize Boo Radley is not a source of danger, which Scout still didn't.
