Whether or not the experiment of the lawyer and the banker has proven that capital punishment is less cruel than life imprisonment is debatable even at the end of the narrative of "The Bet." It has, however, demonstrated that forced isolation is a terrible thing, although the lawyer has had some periods of enlightenment and enjoyment.
In one of his stories, writer Joseph Conrad's character observes, "Meaning depends upon sharing." For the lawyer, all his intellectual betterment becomes meaningless because he is isolated. In the letter that the young man writes at the end of his term, he boasts of all he has learned and vicariously experienced, along with the wisdom he has acquired. Yet, from what he has learned from the writings of many men, he has become cynical about the world and his fellow man in what he terms the vanity of their desires:
You have lost your reason and taken the wrong path. You have taken lies for truth and hideousness for beauty.
Without someone with whom to share his ideas, without someone by whom he can measure himself and his ideas, the lawyer finds no real meaning to life, instead perceiving knowledge and learning as worthless. His years of learning are all vain, empty, and illusory for him because he cannot put ideas to use or share them with anyone or anything. While it is better for the lawyer than if he were to have been put to death, his life now has become a punishment, not a reward.
After reading the lawyer's letter, the banker realizes that his bet has subjected the young man to an agonized life, and he is deeply moved. He feels "so great a contempt for himself" that he lies for hours on his bed because his "tears and emotion kept him for hours from sleeping." His life, too, has become a punishment because of the cruelty he has inflicted upon the once young lawyer.