I think you're right. Marcus wasn't totally a bad guy, but he wasn't totally good either. I'm not sure that he would have taken leadership out of the kindness of his heart, he seemed pretty power hungry even at the end. I think he valued power above everything else. However, what he DID with that power wasn't all bad, like you said. I agree with you, he had the good sense to promote Sean, and keep the sadistic Gatekeeper at bay.