Why did Marguerite hate the Marquis de St. Cyr?

Marguerite hated the Marquis because he humiliated her brother Armand, who was deeply in love with the Marquis de St. Cyr's daughter.

She hated the Marquis. Years ago, Armand, her dear brother, loved Angele de St. Cyr, but St. Just was a plebeian, and the Marquis full of the pride and arrogant prejudices of his caste. One day Armand, the respectful, timid lover, ventured on sending a small poem—enthusiastic, ardent, passionate—to the idol of his dreams. The next night he was waylaid just outside Paris by the valets of Marquis de St. Cyr, and ignominiously thrashed—thrashed like a dog within an inch of his life—because he had dared to raise his eyes to the daughter of the aristocrat.