One motif is the mockingbird itself. It is a motif that appears through numerous characters and it represents innocence, that's why to kill it means to destroy someone's innocence. One example is Jem who goes from being a kid to doing what he does at the end, another is Tom Robinson who goes from being a good man to still being a good man but now accused of committing a sexual crime and tries to escape prison, and the third can be Boo Radley who goes from being that weird guy on the block to doing what he does in the end of the book.