It symbolizes innocence and thus it would be criminal to kill it. Interestingly enough, however, that's exactly what happens, figuratively speaking. Scout and Jem move from extreme innocence to a more mature understanding of the world. The mockingbird is a song bird, one of the characters tells the children, and to kill something that only brings beauty into the world is wrong. The mockingbird is also used to potray morality and integrity throughout the novel.
