Think of it this way, I compare Marcus with Edward when Bella was gone. We know that Edward "holed' himself up in some dirty, nasty place, just trying to cope with the pain of loss. He was a mess, barely able to sustain himself. Marcus isn't evil, he is a vampire that unlike the Cullens, embraces his nature (by preying on humans - seeing them like cattle and nothing else), who is just "wasting" time because Aro won't let him go. With the revelation of Chelsea in Breaking Dawn, I'm sure she is using her talent to make sure Marcus doesn't break with the Volturi since his gift is important to the Volturi. So he does his duty but his heart is not in it. He merely exists, doing his job but finds no joy or pleasure in anything because of the loss of his mate.
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