Why does Atticus return home early and what is tragic about what he said has happened?

Atticus comes home early and requests Aunt Alexandra and Calpurnia's presence in the kitchen. He reveals that Tom tried to escape from prison and was shot to death by the prison guards. Apparently the guards tried to tell him to stop and fired warning shots, but Tom kept running. Atticus needs Calpurnia to go with him to Tom's wife to give her the news.