What are some incidents in chapter 25 of To Kill a Mockingbird that thematically relate to the title? How are they are used and why?

The title To Kill a Mockingbird refers to Atticus's dictum in chapter 10 which states:

"Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird" (90).

The theme revolving around the fact that it is a sin to kill a mockingbird refers to injustices placed on less fortunate or less powerful people at the hands of those who are more powerful. Another way of wording the theme might be that it isn't right to take advantage of other people simply because they are incapable of defending themselves. In chapter 25, the first unfortunate soul is a roly-poly. Scout finds one on the back porch and is about to kill it when Jem tells her not to do it. Jem's reaction is a direct result of witnessing how the town and Mr. Gilmer treated Tom Robinson before, during, and after the trial. Now, Jem sees anything innocent as a victim in need of defending. Scout thinks to herself the following:

"Jem was scowling. It was probably a part of the stage he was going through, and I wished he would hurry up and get through it. He was certainly never cruel to animals, but I had never known his charity to embrace the insect world" (238).

This passage shows that Scout sees a change in Jem's behavior since experiencing the Tom Robinson trial and all of the injustices that came with it. Since Tom Robinson is like a mockingbird in the story, the title and the theme coincide with his trial and with Jem's reaction to it. Scout does ask Jem why she can't smash the bug, and his reply is somewhat like his father's rule about not killing mockingbirds: "Because they don't bother you" (238).
The next incident in chapter 25 that reflects the theme and title is when Dill tells Scout how Helen Robinson took the news of her husband's death. Dill says the following:

"Scout . . . she just fell down in the dirt, like a giant with a big foot just came along and stepped on her . . . Like you'd step on an ant" (240).

The image of a powerful giant stepping on an ant is used to describe how crushed Helen feels at the sad news. Not only that, but if the giant represents society, then Helen is squashed by it because she is then the powerless ant. This image can be directly linked to the theme of mockingbirds as well.
Finally, Mr. Underwood's editorial is described in chapter 25. The references that he makes in his newspaper that also directly relate to the mockingbird theme are as follows:

"Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds by hunters and children . . ." (241).

It is clear that the lesson to take home from the book is shown in the title. It is a sin to kill a mockingbird. In other words, it is not right for the powerful to take advantage of the powerless. It is also useless and wrong to kill or hurt someone who doesn't bother anyone else and is innocent.