The change that both men undergo forms the essence of the short story.  The banker has become financially ruined, to the point where his fulfillment of the bet would break him.  The banker was ready to kill the lawyer in order for him to not have to pay off the debt owed.  It is interesting to see how the banker has changed from his original argument.  Prior to the wager, the banker believed that capital punishment was to be embraced because it was "more humane" than life imprisonment.  Yet, at this particular moment, he was willing to take life of a prisoner, not on the basis of humanity, but rather because he simply wanted to avoid his responsibility to another.  The more profound change is in the lawyer.  His letter indicated that he is willing to renounce everything in the world, declaring life a meaningless void.  This is a vast difference from his initial position which indicated that life in prison is still life, worthy of living.  In the lawyer's time, he has gained proficiency in a great many talents and skills, but can only conclude that life is meaningless.  This was not where he was at the start of the story, but where he is at the end of it.