Did Scout kill the mockingbird or not? Why or why not?
"Remember, it's a sin to kill a mockingbird."
So says Atticus Finch, one of the great moral compasses of American literature. Miss Maudie, Scout's neighbor, elaborates by explaining it is wrong to kill a mockingbird because mockingbirds are innocent and only bring us happiness.
Thus, any character whose innocence is threatened could be a "mockingbird." There are many characters in the novel who could possibly fit this description (including Scout herself). However, the two most common responses to the question, "Who is the mockingbird?" are Tom Robinson and Arthur "Boo" Radley.
There's not a lot of textual evidence to support the idea that Scout has anything significant to do with killing Tom Robinson. However, one could possibly argue that she helps "kill" Arthur "Boo" Radley's innocence in two distinct ways.
At the beginning of the book, Scout "kills" Radley by believing all the false rumors and projecting her prejudices onto him. She refuses to believe he is innocent and believes he must remain hidden for some horrific reason. Like everyone else in town, she is unsympathetic. It is this collective lack of sympathy that makes the innocent Radley a prisoner in his own home. This is a relatively straightforward argument to make.
The second argument is more speculative, but I include it just to get you thinking. One could possibly suggest Scout "kills" some of Radley's innocence (where we read innocence as a synonym for naiveté or inexperience) because she draws him into the life of the town.
First, there is the episode where Radley leaves his house to place a blanket around Scout on the night of the fire. Second, and far more important, is his rescue of Jem and Scout from Bob Ewell. After he rescues the children, he remains at the Finch house, even though he is clearly uncomfortable being away from his own home.
Scout tells us she never saw Radley again after that night, which suggests he went back to being an isolated recluse. However, the fact that she drew him into the life of the town, even if just for a little while, means that she would have killed just a little bit of the mockingbird in his personality.