But to comment on your point: species DO pay for it in the long term when members murder or teratorialism.

Lions are not cannibals. Some lions are cannibals. A successful group of lions cannibals existing (and what a brutal and awesome-in-the-biblical-sense story it is!) does not mean that it pays for the lion species as a whole to have groups of cannibals existing.

In fact, I could only see the “proliferation of groups like this committing atrocities” reach a tipping point for a species - not murdering when this murdering happens will make you cease to exist. So if the species doesn’t have a reason to reach the extreme where this NEVER happens, then it will quickly reach the point where this ALWAYS happens

On an individual level, even in authoritarian situations murder is still immoral and illegal.

Even within the context of power it's still nominally immoral. Stalin did not kill people, he tortured them until they 'admitted their crimes'. That Stalin needed to present the blood-stained admission is very telling.

Even Bloody Mary couldn't just kill however, there had to be some kind of legitimate premise. Heretics, threats etc.. It's how QE1 survived.