`Well, it'd be sort of like shootin' a mocking-bird, wouldn't it ?’

When scout comments `Well, it'd be sort of like shootin' a mocking-bird, wouldn't it ?’,  this refers to what Atticus said earlier about not killing mockingbirds.  Scout realizes that Arthur Radley is a mockingbird.  He hasn’t tried to hurt anyone and was kind to the children.  So, putting him on trial would not be a good thing to do as he was gentle and kind.  Even putting him in the public eye would be a bad thing, as he is mistrusted and even feared by large sections of the public. According to Atticus, hurting him would be a sin.
Putting him on trial would not be a good thing to do as he was gentle and kind.  Even putting him in the public eye would be a bad thing, as he is mistrusted and even feared by large sections of the public.