Marcus to me, seems an enigma. At the end of BD I wasn't quite sure which way he would vote, we don't know enough about his personality to know if he's compassionate (somewhat like Aro) or as power hungry as the other two. I think that the loss of his mate would've been a devastating blow, when we see how viciously the other vampires are willing to protect their mates, however, if this was the case why didn't he just ask Aro and Caius to be released from duty as a member of the Volturi and go his own way or ask them to end his life? It makes you wonder what, perhaps, he owes the other two that he would stick around when he's so dispassionate.
Then again the fact that he's so apathetic could be the reason that the other two don't let him go his own way. The apathy that is in Marcus could provide a good neutral judge who can go with the facts instead of image preservation (Caius) or satiation of curiosity (Aro). As I said, he's an engima and one character I wish had been explored a little further.
Come on and pick me up, somebody clear the wreckage from the blast....-Jack's Mannequin, The Resolution
So I know I'm not alone in a world full of Vampires-Jack's Mannequin, Cell Phone