It is obvious from your writing that you have a very good and wide understanding of many aspects of the Warhammer 40,000 fluff and it truly made this a pleasure to read. This is probably why I got through the whole piece this evening; something I struggle to do with fiction written by fans. I hope this in itself is testament to how much I enjoyed this piece of writing. Very well done.
As a fellow writer I understand the need for feedback, and so I've come up with a few points to bring up for discussion.
First of all, I really like most of your characters, but the traitor Astartes feel a bit off to me. Actually, it's not the Astartes, but the mortals' reaction to such horrors being in such close proximity. These creatures are walking myths and legends of terror that many humans (in my interpretation) do not believe exist. To be confronted by them should make a mortal waver somewhat as they've just witnessed one of the Emperor's former living angels come to murder due to the hatred so instilled by “fairytales.”
Saying this, however, I feel that different writers can have very different opinions on how mortals would react with Space Marines and this interpretation works very well along with the story. It keeps it fast pace and flowing.
If you'd like to see how I see humans viewing Chaos Space Marines I'll share a link to a story I'm currently writing, but out of respect I don't want hijack your thread.
The combat scenes are very well paced, with perfect long stretches of description breaking into quick, brutal meleé to give a good idea of what is really happening, whilst keeping a reader enthralled.
One of the things I liked most about this story was in the third chapter when the Aralee meets with the Marshall. Having Tau converts to an Imperial cult gave me a face that can't be expressed through emoticons, but in a very good and curious way. I'd not thought of such a situation before, and I loved it.
The last point I'd like to make is that the orchestral play scene of The Golden King is one of the most memorable scenes I've read in anybody's stories for a very long time. It's very unique and descriptive.
I'm going to add this story to the Harlequin's Tales sticky. It was truly a pleasure to read.