What are Scout Finch's motivations and her point in To Kill a Mockingbird?

The novel covers Scout's early life, from about age five to age eight. So, she has various motivations for some of the things she does. Consider a few events and how they illustrate some of Scout's character traits.

In Chapter 3, Scout picks a fight with Walter Cunningham Jr. because she thinks he got her in trouble at school. However, Jem steps in and invites Walter to their house for dinner. Scout has a quick temper and is more of a tomboy than other girls her age. When Aunt Alexandra comes to live with the family, she will try to teach Scout to become more of a lady and Scout resists this. 

Near the end of Chapter 15, Walter Cunningham Sr. and a mob of men have come to the jail to harm Tom Robinson. Atticus and Mr. Underwood are watching for just such a thing. Scout, Jem, and Dill have come to see what Atticus is up to. While hiding and watching Atticus confront the men, Scout senses something is wrong: 

This was the second time I heard Atticus ask that question in two days, and it meant somebody’s man would get jumped. This was too good to miss. I broke away from Jem and ran as fast as I could to Atticus. 

Scout then tries talking to Walter Cunningham Sr. In her own naive way, Scout is trying to facilitate some kind of peaceful resolution. So, with the first event, we see Scout's temper and reluctance to become more like a lady. With this event, Scout makes an effort to find common ground and it is successful. 

In the end, Scout learns to appreciate the perspectives of others. At the end of the novel, Scout stands on the Radley's porch, thereby literally and symbolically looking at things through Boo's eyes. This is one of the primary themes of the novel. As Scout matures and learns more about the social divisions of her town, she learns to be more empathetic. In learning this empathy, she learns that it is a sin to kill a mockingbird because it is a sin to hurt the innocent. But note how young Scout is in the novel. She is precocious and smart for her age, but she is usually motivated by naivety. However, these are all learning experiences and the knowledge she gains informs her decisions as she gets older.