I think that Roberts is definitely The One With The Markings (when I type it like that, I keep having Harry Potter 'He Who Must Not Be Named' flashbacks lol. ) His role becomes progressively bigger as the story line continues.
They mention one thing twice, that he was being transferred to a mental health facility. So, I'm almost willing to bet money that maybe not the mental health facility itself but the hospital, Cain General Hospital where Cain Mental Hospital is located in an unmarked area has something to do with the whole outbreak, though maybe only the progression of it.
Also I wonder, since it's been mentioned that people who were smart in death were smart in life, that if in addition to be paranoid Schizo, if Roberts wasn't also extremely intelligent and he's the one creating the different types of zombies like the little ones and Behemoths. That's all I've got for right now though.