In what way are hearts like courts in To Kill a Mockingbird?

Despite Atticus presenting clear evidence in Tom's defense, Tom is convicted of raping Mayella Ewell. Tom feels hopeless. While at the prison, he tries to escape and is shot to death. His bad arm prevented him from climbing over the fence before he is shot.

Mr. Underwood writes an editorial for The Maycomb Tribune.  In it, he writes "it was a sin to kill cripples, be they standing, sitting, or escaping." He likens Tom's death to the "senseless slaughter of songbirds by hunters and children" (To Kill a Mockingbird, Chapter 25).  

Scout wonders about Tom's trial and death after reading the editorial. She reasons that Tom receives a fair trial. Atticus works hard to defend him. A jury of twelve men examine the evidence and then convict Tom. She realizes that, although Atticus fought hard to show clear evidence proving Tom's guilt, "in the secret courts of men's hearts [he] had no case." The jury already made up their minds because the case concerned a black man accused of raping a white woman. The deep prejudice in the jurors' hearts prevent them from doing anything but convicting Tom Robinson.