Aside from Mr. Nathan Radley filling the knothole and Tom Robinson's trial verdict, what other instances lead Jem to view Maycomb as an unfair place in To Kill a Mockingbird?
After the trial in Harper Lee's To Kill a Mockingbird, Jem begins to realize that Maycomb's unfairness is especially expressed in its citizen's prejudices. Though he tries to wrap his mind around these prejudices in a way that may make an ounce of sense to him in order to try and alleviate some of his anger, the fact remains that the town, rather the world at large, is full of irrational hatred that cannot truly be explained away.
In Chapter 23, just after Aunt Alexandra angers Scout by calling Walter Cunningham "trash," Jem tries to comfort her by explaining Maycomb's class differences. Jem says he has given differences a lot of thought since the trial and has finally come to realize that, in Aunt Alexandra's view, what makes a family a "good" family, a family with "background," is education level. According to Jem's interpretation of his aunt,
"Background [means] ... how long your family's been readin' and writin'" (Ch. 23).
Therefore, to help himself understand what Maycomb's society sees as differences, Jem has applied his interpretation of his aunt's opinion to different classes within Maycomb. According to Jem, Maycomb can divided into four different classes of people:
There's the ordinary kind like us and the neighbors, there's the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes. (Ch. 23)
Each of these classes of people are different because of their different education levels, and neither class likes the other. Ordinary people, like the Finches, have substantial education levels, whereas the Cunninghams only have a little bit of education; the Negroes have little education, and the Ewells refuse to be educated at all.
When Scout protests, saying that there is really only "one kind of folks," Jem's retort expresses just how frustrated he has become with society due to his realization of its unfairness:
If there's just one kind of folks, why can't they get along with each other? If they're all alike, why do they go out of their way to despise each other? (Ch. 23)
In Jem's mind, the fact that different groups of society all hate each other is very unfair and something he feels he must rationalize to be able to cope with. Seeing that differences in education level make people different helps him rationalize some of that unfair hatred.