Why was tom robinson shot?

Tom Robinson is shot trying to escape from the prison. The prisoner guard said they were out in the exercise yard and Tom took off running toward the fence. The guards shot Tom 17 times in the back as he tried to climb over the fence. The guard told Atticus that if Tom had two good arms, he would have made it. They yelled at Tom to stop and shot several warning shots before they killed him. Atticus tells Calpurnia that he had tried to tell Tom that they had a good chance on apeal, but he guessed Tom had simply "gotten tired of the white man's" justice and tried to run away. The guard saying that he would have made it if he had two good arms shows that he is held back, or disabled by the arm, which symbolizes how he is held back, or disabled by his race. He is then persecuted because of his disability in both cases, one when he was convicted and the other when he was shot.