A battalion of Space Marines is deployed to a sector where a well known heretic or enemy VIP has been spotted. The leader of this SM battalion has a particular grudge against this guy/gal/thing and will do anything to seek vengeance upon this person. He does everything in his power to find this enemy, maybe even going as far as to accuse all the people in a certain city/planet to be heretics or tainted. Objections from less experienced SMs fall on deaf ears and soon this determined leader's actions increase tensions with other previously uninterested parties. War is soon brought on after more missteps. Many civilians are killed, enemies from other factions show up to take part in this growing fight, essentially dooming the peaceful planet to years of conflict and many of the Space Marines under this leader perish as a result. When all is said and done, it turns out that this enemy VIP was dead long before he/she/it could have possibly done any damage or maybe that person/thing was never in the sector. The "heroic" SM leader realizes his men were correct to question him and that if he had heeded their warnings, the current situation for the people of this planet would be much different. How would the remaining Space Marines handle what has happened?
A Spec Ops: The Line inspired picture. I've been getting a lot of ideas from that game, many of which include other fictional universes doing things that make the character/viewer/player/reader/whatever experience cognitive dissonance and other feelings of discomfort because of the actions they've done. This image by itself was only really an attempt to mimic a piece of promotional artwork for SO:TL using 40K stuff, but this is what was going through my head at the time. I just thought I'd share.
Look at picture. Listen to this. [link]
Garry's Mod and Photoshop