Mhalir is, in fact, thinking about how to fight Hell. It seems like a very hard problem. Convincing Carissa that Asmodeus is actually horrible and helping the Yeerks is morally better and also that she should pick actions according to ethics at all, rather than according to what will get her tortured, seems like almost as hard a problem.
...There is one thing he's noting, which is that in many senses Carissa has an outside perspective he lacks, but there's at least one area where the opposite is true. She...seems to take for granted that Asmodeus' philosophy is true? Or not even be assessing its truth value, maybe, it's unclear, but she at least seems to think she's doomed to go to Hell, and this is also not obviously true.
And one thing he knows, from Earth and from various other civilizations, is that philosophies vary a lot, even within a single species.
There are other countries in this world. Other gods, and presumably their churches have their own philosophies. Unfortunately Mhalir finds all of them terrifying. ...Or, well, at least the Good gods sound terrifying too, and the Chaotic ones are pretty alarming. Maybe there are Neutral - Lawful Neutral? - gods that just wouldn't be interested in interfering with the Yeerks, because they're pursuing their own sideways interests.
<I want you not to lie to me on factual questions> he tells her. <Your motives are your own, and you need not pretend you are cooperating out of anything other than self-interest, or that you do not intend to flee at the first opportunity, and I have no expectation that you will tell me of your plans if I ask. But if, for example, I ask you about Golarion's geography, I want you to tell me honestly what you know. Can you agree to that?>
And he watches her thoughts carefully.