I now actually feel sorry for the poor marathon marine. The fact that he's destiny is more of a curse than a blessing. If he was a mind of a dead soldier (the PiD soldier no less) grafted to a machine, he might be nearly immortal, but he still has the psyche of a mere human. Human minds weren't made to be resurrected, much less last forever. After a few hundred years, if not immediately after being grafted to the battleroid, the marine would become profoundly psychotic. A human mind can only take so much before it goes nuts, and the marine is no exception. Remember all he goes through. Get thawed out, go kill some pfhor, run some missions for Durandal, switch alliances a few times, find out more about himself, then get put in stasis again. If that's all he knows, it's no wonder that he would be crazy.
Rampancy = advanced psychosis? Hmmm...