For millions of years, hominids and their closest relatives lived in small bands of hunter-gatherers. Our psychology was hard-wired for this lifestyle, including coöperation and intense solidarity.
There was, however, no selective pressure to extend these beyond the small numbers in the band. We are not wired for large, diverse societies, or extension of moral consideration and solidarity beyond our immediate community. In this respect, we're like modern chimps, who war with neighboring bands, apparently just for the fun of it.
Nor are we wired for long-term planning or concern. It's only recently that we even became capable of affecting our environment. Through 99% of our history, planning beyond the next season was impossible.
Concern and moral consideration for non-band members; for supra-Dunbar* numbers, and for our affects beyond our immediate future must be learned, ie: indoctrinated, and they remain a thin, easily breached veneer.
We're a civilized species with a Pleistocene psychology; killer apes in suits.