"cruelty is a human concept"
...so is happiness.
The "purpose" of society is not the same as that of an ecosystem. In fact, an ecosystem has NO purpose, it simply is all that has survived from chaos due to natural selection. Society has a purpose, indeed, many of them: justice, peace, security, prosperity, freedom.
Efficiency, stability and resilience are the false promises of, and shameful excuses for, Utilitarianism, a philosophy that was exposed as a cruel hoax over a century ago.
It is possible, even essential, to have options and choices when constructing a society, to deny that is to assume we are the helpless pawns of blind forces. At least, that is the answer you get when you poll the gazelles, and not the lions.