Short-sighted thinking. I posted a small article a comment or two ago. Read it.
Tolerating someone violating the Golden Rule destroys the fabric of society that the said person too relies on, thus unravelling it. Since every individual is driven by natural selection to look out for his or her progeny, destroying this society, in the long run, doesn't bode well for that individual and his or her progeny, over time. To that end, it makes it imperative for successful societies to not encourage entities that destroy altruistic behaviour.
So, pray tell, how did ANY society survive long enough to eventually get the point that they must "look out for each other" and not kill or be killed? You contend that this "golden rule" required millenia to be learned, so how could any non-golden rule society continue past their own lifespans - if they were lucky?