I've read all three books.
When I ask if he has already been turned I mean looking back at everything he did in a new light, long before the stabbing.
Things that he says and ways that he behaves are what I'm talking about.
And I think the Space marines breaking the siege that has held up the guard for so long in the first book is a wonderful bit of fluff, a true glimpse at how hard marines really are, not the wusses that die and are taken out of action by thing like missing limbs and bullet holes in their chests that crop up in some of the more recent novels / stories.
(Apologies for any typos I may have missed, my wireless keyboard hates me)