Why does Jem conclude that Boo Radley stays inside because he wants to? What does he imagine Boo is trying to avoid? (ch 23)

In Chapter 13, Jem states, “I think I’m beginning to understand why Boo Radley’s stayed shut up . . . it’s because he wants to stay inside.” Jem says this as a direct response to the behavior of the people of Maycomb in response to Tom Robinson's trial, and the way people are treated because they're different. He finally understands that maybe Boo is simply hiding away from the ugliness of the town.

Why is Tom's death compared to killing a songbird