Sophia Antonovna exists to provide Under Western Eyes with a reliable witness and testimony. She sees Razumov from the outside, and her description of him is untainted by the hatred that the narrator feels. So the pain she sees is real, though it's not visible to the narrator or to Razumov himself, who believes that he feels only hatred for Haldin and contempt for the revolutionaries he is paid to betray. The only moment in the novel when Sophia Antonovna becomes unreliable is when she claims, in the last words of the novel, that Peter Ivanovich is a remarkable man. Here she becomes another woman deceived by the Great Feminist.
Romance is crucial.
Razumov is trapped in his lies by General T after Haldin implicates him, and he's trapped as well by his feelings of guilt. His love for Natalia frees him in the end to live with Tekla the Samaritan. Romance also motivates the otherwise unimaginative narrator. He is clearly in love with Natalia, though he can barely admit this to himself. His solution is to match the other attractive character, Razumov, with Natalia, and his failure to accomplish this may explain his great bitterness at the end.