One of the messages in Ring Lardner's story is that cruelty has consequences.
Jim is someone who the barber seems to have really admired. He saw him as a funny man who played funny jokes on the people in the town. His jokes had a dark side though and were often aimed at people who couldn't fight back. For example, Paul was a mentally challenged young man who Jim liked to make the butt of his jokes. Jim was also cruel to his own family and targeted an innocent young woman because the man she liked once showed Jim's family kindness instead of letting them suffer at his hands.
It's obvious to the listener that Jim was a cruel man. Even though the barber appears to have enjoyed Jim's humor, it caused pain throughout the town. The barber explains that Jim died recently on a hunting trip with Paul, soon after Jim humiliated the young woman, Julie. Julie and Paul were also friends. The barber—and the rest of the town—believes that Jim was killed when Paul mishandled a rifle.
It seems to an outsider, though, that Jim's cruelty had run its course and he was punished for it by Paul. Because the town only sees people in the way it wants to—such as seeing Paul as a young man who is slow and not a danger—they've missed the signs that he likely killed Jim on purpose.