i feel like the complicated biological explanation may be unwieldy - a look at the history of the Greek city states, or at the modern history of Somalia or Afghanistan, will provide lots of precedents for different factions starting from a mostly shared baseline culture and getting to radically different places in terms of level-of-fucked-up-ness. there are also plenty of historically attested practices (human sacrifice, slavery, institutionalized rape, execution for trivial offenses, going to war for the sake of an excuse to gloriously kill enemies, planned, bigotry-inspired mass murder) that would be more-than-sufficient to tag somebody as evil. there's also no historical shortage of societies that placed more of an evidence on parasitic raiding behaviors and less on any kind of actual productivity. and changing political circumstances, major religious upheavals, moderate geographic dislocations, charismatic leaders, et c. can do a pretty good job of effecting radical shifts in all these areas in the space of a few generations.
maybe like the good guys, the bad guys started out as a culture with a proud warrior aspect, but with a more nomadic lifestyle - over time, productivity became de-emphasized in favor of warrior glory, to the point where eventually they became a fully nomadic raider culture, with some emphasis on killing in battle as a glorious thing to do. being nomads, they're only recently arrived to this particular area, and having had a couple more lean years they're even more desperate than usual.
or there are other options. the early violent-expansionist phases of major world religions, the histories of various fundamentalists cults and sects, the rise of various totalitarian regimes, or just precedents of in-certain-respects-very-nasty historical cultures like the Vikings or the Aztecs, could all be good points of departure.
η: hell, to see how a couple of generations of ordinary human political forces can turn basically identical cultures into cultures of drastically different degrees of badguyness, one need only look at Korea.
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maybe like the good guys, the bad guys started out as a culture with a proud warrior aspect, but with a more nomadic lifestyle - over time, productivity became de-emphasized in favor of warrior glory, to the point where eventually they became a fully nomadic raider culture, with some emphasis on killing in battle as a glorious thing to do. being nomads, they're only recently arrived to this particular area, and having had a couple more lean years they're even more desperate than usual.
or there are other options. the early violent-expansionist phases of major world religions, the histories of various fundamentalists cults and sects, the rise of various totalitarian regimes, or just precedents of in-certain-respects-very-nasty historical cultures like the Vikings or the Aztecs, could all be good points of departure.
η: hell, to see how a couple of generations of ordinary human political forces can turn basically identical cultures into cultures of drastically different degrees of badguyness, one need only look at Korea.