The moral of the story is that people should treat one another fairly, as equals, and with respect, and not be blinded by prejudice towards others of different race, colour, background or creed. The author shows the nature of a community where such prejudices are rife, and the serious consequences of this.
In the Southern town of Maycomb in the nineteen-thirties, the main form of prejudice is racism. Lee dramatises this in the form of a trial of a black man wrongly accused of rape, and in so doing she succeeds in showing how prejudice is absolutely entrenched in society, staining even the justice system.
However, racism is not the only form of prejudice that the novel addresses. There is also much time devoted to examining class prejudices, particularly with the figure of Aunt Alexandra who appears quite comically obsessed with family heritage and tries to prevent her nephew and niece from mixing with who she regards as unsuitable people, whether black servants like Calpurnia or poor whites like the Cunninghams. People who are seen to be odd, like the reclusive Boo Radley, are also discriminated against.
Lee puts the moral of her story into action with the main characters, Atticus Finch and his two children. Atticus demonstrates an ability and willingness to cut right through social and cultural prejudices and to judge an individual – any individual – entirely on his or her own merit. Even more important, he passes these values on to his children. They learn both about the existence of prejudice and also how to overcome it, thanks to his example. The lesson is summed up at the close of the novel when Atticus remarks:
Most people are (nice), Scout, when you finally see them. (chapter 31)
To ‘finally see’ a person means to cut through the layers of prejudice and preconceptions that exist in society, as amply demonstrated in this novel. (We should also note, however, that Atticus says ‘most’, not ‘all’ people; some, like the utterly mean, vicious, and despicable Bob Ewell are seen to be beyond the pale.)
The novel, then, follows the young Scout and Jem’s journey to maturity and understanding and in this way Lee successfully illustrates the moral purpose of her story.
I believed the moral of To Kill a Mockingbird was "It is sinful to take advantage of or destroy something weaker than oneself, just as it is a sin to kill a mockingbird.