In the short story "The Bet," how does the banker feel about himself at the end of the fifteen years?

In Chekhov's story "The Bet," the banker ends up feeling ashamed of himself. 

At no other time, even when he had lost heavily on the Stock Exchange, had he felt so great a contempt for himself.

The banker has a number of reasons for feeling this way.
Probably most importantly, he is ashamed of the fact that he had been considering murdering his prisoner in order to get out of paying him the two million roubles he had won and richly deserved. "Poor creature!" thought the banker, "he is asleep and most likely dreaming of the millions. And I have only to take this half-dead man, throw him on the bed, stifle him a little with the pillow, and the most conscientious expert would find no sign of a violent death." The banker had even been planning to allow one of his servants to be blamed for the lawyer's death and most likely sent to Siberia. "If I had the pluck to carry out my intention," thought the old man, "Suspicion would fall first upon the watchman."
The banker is also ashamed of the fact that money has become such an obsession with him that he can hardly think of anything else. The lawyer's letter in which he renounces the two million roubles serves to make the banker aware of the vast spiritual difference between them. 
The banker feels responsible for the emaciated condition of his prisoner. After all, it was the banker who initiated the bet fifteen years ago at his big bachelor party. He started the whole thing when he said: "It's not true! I'll bet you two million you wouldn't stay in solitary confinement for five years."
The banker feels ashamed of being the de facto jailer of a man for fifteen long years in solitary confinement for no real purpose. "And now the banker, walking to and fro, remembered all this, and asked himself: 'What was the object of that bet? What is the good of that man's losing fifteen years of his life and my throwing away two million? Can it prove that the death penalty is better or worse than imprisonment for life? No, no. It was all nonsensical and meaningless. On my part it was the caprice of a pampered man, and on his part simple greed for money ...'"
No doubt the banker is ashamed of the fact that his prisoner was, in effect, making him a gift of two million roubles when he should have been giving the two million roubles to the prisoner.
The banker must realize that he was only showing off before his important assembled guests when he offered to bet two million roubles without the prospect of winning anything tangible in return. He is not only ashamed of himself for his present weak and treacherous character, but he is ashamed of the ignorant, vainglorious, materialistic man he was fifteen years before. The lawyer may have lost his youth and health, but the banker has lost his soul.