As groups of social animal species in nature (e.g. wolves, lions, many primates) defend territories against members of neighbouring groups of the same species and if necessary even fight and kill them, so does Homo sapiens to a far greater extend than obvious at first sight:  In native peoples' societies (Indian tribes of the Great Plains of North America for example), the warrior as an honoured member was admired for killing the enemy warrior of the hostile neighbouring tribe. As well as bringing home slaves and women from the other society in which often the same ethical principles were present. The european settlers and the white man called these people "savages". They did not see that also in their own "civilized" societies the soldier who kills an enemy is considered a hero.