How does Harper Lee show that Tom Robinson's treatment is cruel and unfair?

Lee shows that the treatment of Tom Robinson is unfair in many ways. 

First, there is a mob of people that comes to harm Tom Robinson for being on trial.  The injustice of this comes to light further when we consider that the trial did not even take place.  So, a bunch of white men have come to harm or kill him based on hearsay.  This shows pure hatred and blind racism.

The trial itself also shows the treatment of Tom Robinson is unfair.  Even though Atticus gives an incredible defense of Tom, the jury is not moved at all.  To put it another way, he is guilty before the trial even begins. Jem says it best when he says "it is not fair." Here is a quote:

“It was Jem’s turn to cry. His face was streaked with angry tears as we made our way through the cheerful crowd. ‘It ain’t right,’ he muttered, all the way to the corner of the square where we found Atticus waiting.”

Finally, Tom dies in prison.  We do not know the details, but he is shot by guards, who say that he tried to escape.  Hence, the story of Tom Robinson ends even more tragically.