The Golden Rule, or in other words, the Principle of Reciprocity. Break it, and you'll have real-world consequences that are to the detriment of any organised social structure.
Cheers!
(Isn't that what you atheists complain about, that God isn't just, but is unaccountable? So why comfort yourself with fairy tales about "recompense" of dictators when it doesn't even happen here in "the real world" ?)
Cheers!