I think that if a society today were to reintroduce slavery, crucifixion and a number of other aspects of Roman or Greek society this would reasonably be described as a reversion to barbarism.

On the other hand it is undeniable that highly civilised societies in modern times have done things which would have appalled Genghis Khan.

Basically we use words like "civilised" and "barbaric" as rhetoric to indicate how disgusting we find some of the things our contemporaries do, in the hope perhaps that this may convey that feeling to people living in such societies.