What you see in Entropy Zero is a character that has been given the gift of the protagonist
OTA's mindset is contrary to working against the combine, they think, have their own independent thought processes and such, but the MMRs keep them in check
It'd be a case of seeing how psychosis affects the character you portray, in the event it happens (during captivity and/or lack of Combine Lean).
Regarding your comments,
@MaXenzie:
You lose part of your character and in exchange, roleplay something else. Your character is half dehumanized, surely he's not a full fledged soldier but he's augmented and brainwashed.
I think a roleplayer should be able, with experience, to manage and properly portray any character they're given - People roleplay zombies and antlions too, these are more advanced than them of course, in a way that they have thought processes and opinions and morale about their performance and the one of those around them, their enemy, their allies and the Combine itself.
Just because you're not purely human it doesn't mean you're a robot, that's a misconception everyone has made for themselves in this community for God knows what reason (past iterations and the likes?).
On the topic of adversary roleplay, well, humanity against the Combine is quite literally the whole premise around Half-Life 2, while I understand your point we're not aiming for this to become a purely deathmatch server, you can stay at your base, walk around the map and stuff. You can perform non-combat related roles.
Once you choose to join the Combine, you know full well you will eventually end up as a grunt (or RL, leading others into the freakshow). That's a choice you make, a path you may experience. Each path has an end. Regardless, the whole "this will end your character" is a bad conception of the case, as I said, you just pass onto roleplaying it differently; your loyalty unquestionable, and you are more professional, but still a character you can play.
However, everyone has their likes and dislikes, nobody will force you to join a side you dislike.