Why does Don Osvaldo Tomitz struggle with feelings of guilt throughout the novel, A Thread of Grace?

Don Osvaldo Tomitz, a Catholic priest in Sant'Andrea, struggles with his inability to give absolution to Werner Schramm, who confesses that he has murdered 91,867 people. Eventually imprisoned and tortured by the Nazis for helping the Jews, Tomitz dies in jail with Werner Schramm by his side, administering last rites and communion, and finally ending Tomitz's suffering.