In many ways, this novel is a "coming of age" novel. Scout learns many moral lessons throughout the story and she has grown up a lot by the end of the book. If you are looking for moral lessons in this novel, I suggest you concentrate on the advice given to Scout and Jem by Atticus. He is always imparting wisdom to his two children, and many of his lessons take the form of metaphors, if you are looking for particular figures of speech.
For example, when Atticus instructs Jem and Scout to stop tormenting Boo Radley, he says that they should not be:
"...putting his life's history on display for the edification of the neighborhood.."
This is a metaphor that means they should stop spreading rumors and stories about him that are not true and stop prying into his sad life.
He also advises Scout to:
". . .climb into Jem's skin and walk around in it. ."
In an effort to explain to Scout that she should try to understand her brother better. Another metaphor.
Scout realizes at the end of the novel that arresting Boo Radley for killing Bob Ewell while he was protecting Jem would be "like killing a mockingbird." This is a both a simile and a moral lesson. Boo is like the innocent mockingbird, harming no one, existing only to sing a beautiful song to entertain people.
This novel is packed full of similar moral lessons - you just have to skim through a few chapters and they will jump out at you! (ha ha)