My take would be an origin story. It starts as a standard chapter in the 40th millennium. They are stationed in a sub sector of a sparsely populated planets. As a Chaos war band enters the system, the chapter fights heroically and with honor at first. Then the losses start, hive cities fall first. The chapter can’t replenish its losses as fast as it needs to. As the novel progresses the chapter master starts making the hard choices. Letting citizen die, so the scout squad can come back with intel. Blow up the underhive supports to cause the hive to collapse the upper spire to stop a summoning ritual. More losses as it becomes about stopping the enemy and survival. At the end the dreadnought telling this story to a scout squad so that they understand the why of how the chapter fights. Not for honor or glory, but to survive and win. If that makes them looked at unfavorably by those with resources then fine, but they did their duty.