a bad person

[abaris]

Why then do we call people bad when they break such a rule, other than they do something which is arbitrairy forbidden by previous generation. 
(for instance look at: cultural differences)

It's not as if this was a new subject. But, we as a social species, just as every other social species, need some rules for living together. That's the base of our morality. And there have been times when things we consider evil were accepted and encouraged as long as it was directed towards outsiders. Such as Hitler's Germany, Stalin's Russia or Pol Pot's Cambodia, to name only a few.

It will happen again, of course, anywhere, anytime.