I think that, as you say, the problem you are having here is that the word you want to use (barbaric) relates to a group of people said to have the trait of behaving like that (ie barbarians) and as you are finding, once you are among such people (eg the Berbers) they do not, of course, all have those traits as described!
So how about choosing words that ‘describe the act’ rather than ‘attributing that kind of behaviour to a group of people’?
For example, you could use ‘heinous’ which means ‘unconscionably wicked’ (without ascribing that to any particular race).
Barbaric captures the feeling of: primitive, cruelly unthinking, survival based, acting without thought of consequences to the other. Or, the ‘tribal’ mores are seen as being more important than the rights of the individual.
Primitive, cruel, unfeeling, uncaring are all words you could use.
A ‘barbaric act’ is subtly ‘something we wouldn’t do’ - we are distancing ourselves socially from another group and saying we are more civilized than them.
To be honest I really hate the kind of newspaper reporting and writing that does this - that takes one group and contrasts it with another, making the first one bad.
We are all capable of ‘barbarian acts’ and we may need to reserve the right to do those, should the need arise. The veneer of civilization is thin as putty and paper.
I strongly disagree with one answerer who feels that the ‘present day use’ of a word is ‘distanced’ from its original. When I heard the word ‘barbarian’ and you mentioned ‘berbers’ I immediately thought of ‘the Barbary coast’ - it just popped into my mind - even though I have no idea where that is - it instantly arose from my sub-conscious. All the history and meaning of words remains in our subconscious and is a part of the meaning, when we use those words.
Try to find words that describe the act itself - without comparing it to groups who ‘act like that’ or ‘don’t act like that’ - which is, after all, supposition, and probably always will be, subtly or unsubtly - racist.
If you want a word a bit like ‘primitive’ and ‘generically barbarian’ how about ‘atavistic’? Which is a throw back or echo of more primitive traits.
‘Atavistically brutal’ is my suggestion.