Atticus said that Tom Robinson attempted to escape because he wanted to take matters into his own hands, and was tired of white men deciding his fate for him.
Tom Robinson’s escape is one of the saddest parts of the book. We are rooting for Tom throughout, and saddened when he is convicted. It is obvious he could not have committed the crime he was accused of, because he is physically incapable of attacking Mayellla Ewell in accordance with her injuries. We know that racism is the reason he lost the trial.
Atticus explains why Tom ran for the fence in chapter 24.
“We had such a good chance,” he said. “I told him what I thought, but I couldn’t in truth say that we had more than a good chance. I guess Tom was tired of white men’s chances and preferred to take his own. Ready, Cal?” (chapter 24)
Tom made his own choice. He wanted to end his life on his own terms, because he had no other way. Tom has faced a life of racism, and does not want to face white men making decisions for him anymore. He took the decision away from them. It was really the only power he had.
Atticus comes home during the middle of the day while Aunt Alexandra and Scout are hosting the missionary society meeting at their home. In the kitchen, he tells Miss Maudie, Aunt Alexandra, Calpurnia, and Scout about Tom Robinson's death. During the exercise period at Enfield Prison Farm, Tom had taken off running for the fence. The guards had yelled at him and fired warning shots, but he didn't stop. Just as he went over the fence, they shot to kill--filling him with seventeen bullets.
Atticus wants Calpurnia to come with him to give the news to Tom's wife, Helen. Although Atticus appears in control, his sister Alexandra knows that "it tears him to pieces."
He explains that he had tried to give Tom as much hope as he could, telling him he thought they had a good chance of winning the appeal that would overturn Tom's conviction. But Atticus was unwilling to over-promise. Knowing the racism that courts and juries in Alabama were rife with, Atticus understood that the solid case they had proving Tom's innocence might not be enough. So, Atticus said, Tom decided to take his own chance rather than trust his fate to "white men's chances."
Tom's decision is later clarified in Maycomb's newspaper in an editorial written by Mr. Underwood, its owner.
Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men's hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed.
This is no doubt what Tom realized, and why he "broke into a blind raving charge at the fence and started climbing over." Tom knew that he was not likely to win an appeal to the "secret courts of [white] men's hearts."