The mood in To Kill A Mockingbird can be considered dark. One quote to support this analysis would be found in chapter nine. Atticus states to his children that he must try to win, even though he realizes that he lost Tom Robinson's trial a hundred years ago:
Simply because we were licked a hundred years before we started is no reason for us not to try to win.
Truly, it has to be disheartening to Atticcus to try and defend a black man who is already considered guilty before he has a fair trial. Going into a trial with the knowledge that Maycomb is a racist town has to be discouraging. Atticus realizes that an all-white jury is going to find Tom Robinson guilty. This creates a somber, dark mood in the story.
In chapter eleven, Atticus shares with his children that he knows he has lost before he even begins the trial:
It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do.
The fact that Atticus is working so hard to prove Tom Robinson's innocence, yet he knows that his work will be in vain, leaves the reader with a hopeless feeling. Atticus knows that Tom Robinson will be found guilty. This creates a dark mood for the reader. Unfairness is depressing. That is why Jem loses faith in humanity when Atticus loses the trial. Scout cannot help but notice Jem's change in mood when Judge Taylor polls the jury for a verdict:
Judge Taylor was polling the jury: "Guilty... guilty... guilty... guilty..." I peeked at Jem: his hands were white from gripping the balcony rail, and his shoulders jerked as if each "guilty" was a separate stab between them. (21.50)
To Kill A Mockingbird is a novel with a dark mood. The reader also loses faith in humanity. When Tom Robinson is killed, the reader is despondent. The unfairness of it all causes the reader to grieve right along with those in Maycomb who believed in Tom Robinson's innocence.