There's a lot more going on there than immediately springs to mind.

Humans are a tribal species by nature. We are not physically suited to act as lone predators as tigers do. However, by co-operating we are greater than the sum of our parts. One human is prey to a tiger; a group of humans working together can predate tigers. This means that it's in the self-interest of any individual to protect the health and nutrition of the others. It's far more effective to share the limited spoils of a meagre hunt today rather than for three of your hunters to starve, thus reducing the chances of a successful hunt tomorrow. When you add in that most tribes would be based around loose familial groupings, there's also an emotional bias in favour of protecting its members. None of this is ‘nice’, it's a practical and subconscious response to external pressures. So is killing the tribe next door in order to skew those pressures in your favour.

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