: I found it hard to buy that Mickey would betray the team he's served with for
: years just like that. Yo, I didn't sign up to kill other humans so imma
: gonna join the guys who kill other humans and killed the Rookie too
: because I failed to save the Rookie from getting killed by the guys I'm
: now defecting to...

Well, I don't think it's "just like that", but this is an area where a full novel might have alleviated some of your issues. We see that losing the Rookie hurts the team, and Mickey especially, but we don't spend a lot of time with Mickey, let alone in the character's head, to see where things actually "turn".

Betraying his teammates obviously hurt him—it's why he's so willing to be killed after Buck defeats him, and I think why he initially justifies his betrayal with the standard Insurrectionist platitudes.

: On a side note, that's two more entries to add to the rapidly growing list of
: defected Spartan-IVs. I'm sure all those UEG taxpayers appreciate their
: money going to a good cause.

Well, it makes sense. You've expanded the pool of Spartans dramatically, and you also have a shifting political landscape where "become a god, kill some aliens" isn't necessarily your only job description. For all of ONI and branch brass' protestations, not everything is going to be screened out of the candidates, and they're still highly vulnerable to people becoming disillusioned with the UNSC/UEG while in the service.