Kind of sounds like what is happening now in the west under our current financial/economic system, albeit slowly by manufacturing crisis's and health problems to whittle down the population.
>To make a somewhat successful "common good" society you first need to exterminate large swatches of the population
It sounds like you are conflating "common good" with authoritarianism.
Common good happens at the grass roots level and then spreads by consensus. Doesnt mean it will happen on a large scale, esp if there are larger forces at work.
It seems like this is a population size problem to me. Or at least a concentration of large populations in a small area (Cities). Maybe the trick is to spread out the population a bit more and prevent areas from becoming over populated somehow.
I don't have the data in front of me right now, but there are some sources that say when groups become to large people vie for attention which causes the most egoic people to try and dominate the areas that they are in.
Thus this creates social circles that try to gain power for powers sake and do not contribute to society.