Actually, the concept of war is a running theme in the manga as well. Lavi and Bookman frequently bring up the idea that humans are always killing one another, always starting wars for seemingly trivial reasons. Based on Sheryl Camelot's actions in "Flower of Evil" (158th Night), it would appear that the Earl and the Noah are exploiting this tendency to cause the deaths of even more humans than the Akuma could manage alone. And, of course, as you pointed out, in a war, both sides believe they are righteous in their cause.