Andre could have killed Ferrer's character, near the end of their epic sword fight in the theater building but he paused and threw the sword into the stage floor. What ceased his quest for revenge against the man who killed his friend? What happens to Marquis after?
shareAndre asks that very question, doesn't he? I think it's because suddenly a subconcious feeling of fraternal connection happens to him. And indeed, his adoptive dad explains that he is not the son of the Count de Gavrillac, but of the Marquis's dad. This leaves Andre very happy, because he realises that he is not Aline's brother after all, and he is then free to marry her.
I guess the Marquis just scuppers away... I don't know what the novel does with him.