Some stories are better told than others; nevertheless, some concepts the authors try to convey seems universally true.
Slayer, Claymore, Aflame Inferno, Full Metal Alchemist; and as I think back, other mangas I have read, liked, and admired are nevertheless, under the same catagory. These stories start small, give first a snap shot of the lives of protagonists, and then, enlarge the camera angle. A larger picture shows us the conflict between human being is magnified such that nations are at war. Amid the turmoil, there is no middle person, no justice; winner writes history, survival of the fittest.
Human's thirst for power, the desire of money, prestige, will, love, hope. Anything can be the motivation, if you let it. I now agree what he says, 'We are always at war; life is never fair; and that, there is no answer to the question, does the ends justify the means.' In fact, that is a wrong question.