A turning point, by definition, follows a set of plot points that make up the rising action. As such, there are no turning points at the beginning, as the early section of the novel leads up to turning points specific to a particular character or set of characters, theme, plotline, etc.
The novel's central climax or turning point relating to the theme of good and evil occurs in Chapter 28. Bob Ewell attacks Jem and Scout as they walk home from the Halloween pageant. Boo Radley comes to the children's defense, and Ewell is found dead under an oak tree. Sheriff Tate writes in the official report that Mr. Ewell died by falling on his own knife, however, Boo may have killed Ewell (the novel makes it unclear).
As part of the falling action, Scout recognizes Boo as the man who saved her and Jem. Scout treats him like an old friend in an affectionate and heartfelt moment, greeting him with a "hey, Boo." She walks Boo home and never sees him again, but the experience teaches Scout a valuable lesson. Boo is not the dangerous degenerate that Maycomb society believes him to be. It occurs to Scout that Boo had been the person hiding gifts for herself and Jem in the tree. For Scout, this is a lesson not to accept societal judgments at face value.
Going back to the rising action associated with this theme, the lead up to Boo Radley's true nature being discovered features him leaving presents for the children and folding and fixing Jem's pants after he sneaks into the Radley's yard. In the case of Bob Ewell, his violent nature is featured when Atticus proves that he attacked his own daughter Mayella during the trial. After Atticus humiliates him in court, Bob Ewell publicly threatens him. This provides motivation for Ewell to attack Atticus' children in revenge.