why is it ironic that Zaroff reads Marcus Aurelius's work?
I am going to assume that you are referring to this line:
"In his library he read, to soothe himself, from the works of Marcus Aurelius. "
I am also going to assume that the work Zaroff is reading is Aurelius's personal reflections that have since become known under the title "Meditations." Aurelius is considered the last of "The Five Good Emperors," a phrase coined by Machiavelli. He is not being sarcastic or ironic at all. Aurelius really was a good, talented, and very smart emperor. He earned the love and respect of those that he ruled over. His "Meditations" are his self reflections on how to be a better person. They emphasize self reflection and judgment, maintenance of ethical principles, and a focus on being a good man. His writings also seem to reflect an emphasis on the good of the community being more important than the good of the individual.
It's ironic that Zaroff reads Aurelius, because Zaroff is basically nothing like Aurelius or anything like what Aurelius wrote about. Zaroff is selfish. He believes in self gratification. He doesn't care about the feelings of his fellow man. He for sure doesn't care about the life of his fellow man. At no point would I call Zaroff a worthy, honest, fair leader. A sadist yes. But for Zaroff to even contemplate himself being on the same plane as Aurelius is comical.