Karenin considers the duel

Karenin's one weakness is that he cannot bear a woman or a child weeping; his secretaries warn petitioners not to cry if they want their case heard. Anna's tears in the carriage produced the mental perturbation he hid by stopping every sign of life in his face — the deathlike look she noticed. Alone afterwards he feels not grief but relief, like a man whose long-aching tooth has been drawn, and concludes that she was always depraved and that he, having made a blameless mistake, has no cause to be unhappy. His one question is how to clean off the mud she has splashed him with. A duel he examines at length, admits he would never fight, and rejects on the ground that his friends would stop it and the gesture would be false. Divorce would require proofs the law demands and refinement forbids, and would leave her free for Vronsky. Separation the same. What remains is to keep her, hide it, stop the affair, and — unacknowledged — punish her, a course he then finds is also conformable with religion.

Tolstoy lets Karenin reason his way to cruelty by impeccable steps, then notes that the religious justification arrived after the decision. The drawn tooth measures exactly how much the marriage was worth to him. The duel passage is the novel's fairest treatment of him: he is honest enough to identify his own cowardice and dishonest enough to dress the conclusion as principle.