According to Vronsky's guide to life, husbands are foolish creatures that stand in the way of happiness. This view is turned upside down by Karenin's noble and selfless actions.
Vronsky regrets that Anna's last memory of him is the shameful one of him weeping helplessly before Karenin. The love between Anna and Vronsky that Vronsky thought was fading has suddenly come back in full force, and he feels doubly ashamed by his own behavior.
He thinks of all this while standing on Karenin's front steps, and finally the hall porter asks if he would like a cab.
Vronsky goes home and tries to sleep, but he has a hard time. (Note: in his life before Anna, he always fell asleep quickly and easily.)
He thinks of Anna, of his shame, and believes that he is going mad. He thinks about life apart from Anna, and decides that none of it matters. All he has is Anna. And now that he has humiliated himself before her, he can think of nothing but their lost happiness.
He takes out his revolver, presses it to the left side of his chest, and pulls his trigger.
He sees blood and realizes he shot himself. He chastises himself for not killing himself.
A servant finds him and runs for help. Vronsky's sister-in-law, Varya, arrives soon afterward to nurse him.