War and peace.
I am not a medieval military historian, so I'm not sure what soldiers back then went through. In addition, most books detailing pre-gunpowder battles tend to focus on tactics and weapons, not so much on the participants (not the grunts anyways). With that said, a lot of my inspiration came from the experience of modern soldiers. The Things They Carried, by Tim O'Brien, is a great reflection by a veteran. This novel really gets the struggles of a veteran trying to adjust to civilian life, and the comradery he missed.
Eugene Sledge wrote about his journey from WW2 to post-war America in his two novels, and portrayed in HBO's The Pacific. Interesting thing about Sledge was that he distanced himself from the military when he returned; this guy avoided veteran reunions until thirty years later.
As for the combat mentality, Evan Wright's Generation Kill (and the TV series) describes it pretty accurately. I also took pieces of Wright for Madura's character.
Well, plague victims tend to have a very high K/D ratio.
Only if you count teamkills.
