Why did Makar disclose that he had killed the merchant?

"God Sees the Truth, But Waits" is the story of a man, Ivan Dmitrich Aksionov, who is imprisoned in Siberia for over twenty years for a crime he did not commit.
He tries to make the best of his life as a falsely imprisoned man. He devotes his life to God and becomes a friend and confidant for the other prisoners and the prison guards.
Some prisoners, including Makar Semyonich, are transferred from another facility. As he hears them talking amongst each other, Aksionov becomes convinced that Semyonich is the man responsible for the crime he is accused of committing. But he keeps this information to himself.
Semyonich attempts to dig a tunnel that will free him from the prison. The guards discover it and question Aksionov about it, but he refuses to tell them what he knows—partially out of fear of Semyonich, but also out of a lack of certainty that he actually saw what he thought he saw. Semyonich is so moved by Aksionov's act of protection that he freely admits to killing the merchant—both to Aksionov and the authorities. This leads to Aksionov being cleared of all charges, but he dies before he is released.