I'm starting to regret being an ass in the comments. To be perfectly honest, I was kinda looking forward to their being Marines in this. Could have been fun. To make the argument for them, just because they know no fear, doesn't mean they can't be killed. Strictly speaking, Inquisitors are supposed to be able to look at the most fowl abominations that ever crawled out of Nurgles asshole and say "Meh, Imperial Archives say that there are worse things out there." Hell, a Commissar showing his fear is punishable by death.
Remember, Courage isn't having no fear, it's doing what needs to be done, regardless. There is no better example than Caiaphas Cain, who pretty much spent his entire military career wanting to curl up into the fetal position and order his men to fetch his mother, but, at the same time, was the most highly decorated war hero in the Officio Perfectus' history.
Long Story short: I think all of these people, even Space Marines, do have fear, they are just trained not to let it show.
Plus, I'm pretty sure that there are people wanting to play things that aren't Inquisition based.