Disclaimer: Halo was created by Bungie and owned by Microsoft; Red Vs Blue is property of Rooster Teeth.
Agent Maine was dead, as far as the world was concerned. All that made Agent Maine human had long since been discarded irrecoverably; he had ceased to exist. No longer the trained Freelancer Agent Maine had been, it (for it was no longer possible to consider Agent Maine to be a person), was a feral animal. Maybe a smart animal, but not a man. A man would've displayed something; grief, regret, pride, anything, when it killed its comrades in order to attain their AIs and armor enhancements. A man would've shown emotion as it massacred its way through platoons. A man would not have blindly, instinctively rampaged, attempting to sate its hunger for power. In short, Agent Maine, the man, was dead. The Meta, an insane, vicious monster, was all that's left. A sick parody of a man serving as an example of all that was wrong with Project Freelancer.
At least, that's what the world thought.
In reality, Agent Maine was in their all along. At least part of his sanity was left, locked in to bear witness to a path he had chosen. A little bit to feel for all the suffering the Meta had caused. A little bit to wonder why he wouldn't, why he couldn't stop. A little bit to deal with the voices of the AIs, screaming inside his head. Agent Maine was alive, but if he was able to say anything more than animalistic growls, he would've wished for death. He was aware that now he was chasing a floating bowling ball for... some reason. With Wash. Why had the same Agent that had tried to stop the Meta before agree to work with them? Agent Maine didn't care; he just wished someone would end it.
The Doctor had surprised him, overloading his time-distorter. That should've given then enough of an opening to finally end it, but the bumbling morons didn't. Wash hated Doc because he was an idiot. The Meta hated Doc because the Meta hated everyone. Agent Maine hated the Doc because he didn't seal the deal.
So they were in a desert, tracking down heat signatures. Maine was disappointed that the aliens were not all they were cracked up to be; he thought they might've been able to kill him. So Washington tracked the Epsilon Unit, for whatever reasons Wash had. The Meta tracked the Epsilon Unit because it wanted it. Doc and Maine followed; Doc because he was a prisoner (although a distractingly cheerful one) with a gun to his head, Maine because he was a prisoner in his own body.
And they kept tracking Epsilon. Part of what was left of Maine hoped that whoever had it, would manage to finally put an end to things. Another part knew that never would happen. Another part sarcastically reminded himself you wanted to be the best of the best, Maine. And look where that got you.