I'm not sure what sections you're referring to in particular, but there's a lot of thematic stuff in that novel that sets the characters up as being in a believable frame of mind to do what they do, and what the novel is about.
I'd say emotional state rather than frame of mind, but I'm not sure that would be an accurate phrase for space marines. They're all 'philosophically' damaged, or damaged in their ideals/beliefs/purpose for keeping going. I imagine you know the point of the book's story, but I don't want to spoil until you get there. The author is creating depth to the characters so you can see why they end up making the choice they do.