The bias is in the presentation of only the banker's perspective. Except for the letter left by the lawyer, the reader is not privy to this man's thoughts.
In this highly psychological short story, having engaged in the bet has reduced the banker to contemplating the murder of the lawyer so that he does not have to pay the "two million." Apparently, the lawyer has had his sanity threatened by his solitary confinement, bringing into question the lawyer's assertion that life on any terms is better than death.
As the fifteenth year draws to its conclusion, the banker worries how he will pay the debt since he is no longer extremely wealthy. He also considers the foolhardiness of such a bet:
On my part it was the caprice of a pampered man, and on his part simple greed for money....
The banker peers into the room where the lawyer has been for the last decade and a half and finds the lawyer motionless. He taps at the window, but there is no response, so he breaks the seal on the door, uses the key, and enters. Still the inmate does not stir.
At the table, an emaciated man sits; he is asleep. Before him lies a letter written by this solitary man, a letter that renounces his fellow men:
....I tell you, as before God, who beholds me, that I despise freedom and life and health and all that in your books is called the good things of the world.
The lawyer writes further that he has learned that the exploration of music, art, thought, feelings are all incomplete without human relationships. But, the banker demonstrates only his self-interest as he locks away the lawyer's letter as proof of the lawyer's having denounced the money.
In the bias of the author for revealing only the banker's thoughts, it is left to the reader to determine the meaning of the emaciated and aged appearance of the lawyer, as well as what has motivated him to write such a letter.