Order, disorder, cruelty
Cruelty is invented by the weak to fend off the strong [analogous to poison], among highly related members of a particular population. The long term benefit is the building of barriers, leading to speciation. This is reflected in civil wars, such as what happened between 1944 and 1949 in Greece, or more recently in the Balkans. In biological terms this prevent pan-mixing, resulting in formation of recombinants of various types, that, although highly related to their origins, can easily mark themselves off, creating sub-populations. In terms of cultures, this is the way groups are created that invent a history that they do not wish to share with others, thereby creating efficient barriers to inter-cultural marriages. The closer the groups are, the more cruel they need to be to split from each other.
The text of the presentation Order, Disorder, Cruelty is in French, and the slides of the presentation are in English.