The word barbarian came into English from medieval latin, (Barbarinus) this word came from ancient Greek (BapBapoc) The ancient Greeks used the word liberally and generally to describe people or tribes that did not speak Greek. The Greeks used the term as they encountered different cultures including the Egyptians, Persians, Indians, Celts, Germans, Phoenicians, Etruscans and Carthaginians.
The term has also ben used historically to describe the Vikings, Mongols and the Goths. The "Normans" also were commonly called barbarians during their invasion of England. Idiomatic usage of the term barbarian (brutal, cruel, warlike, insensitive), would put such people as Hitler and Stalin as being Barbaric.
Attila the Hun is one of the best-known barbarians. Adolf Hitler, although he was a despicable man, was not a barbarian. With regard to the alleged "idiomatic usage" of the term "barbarian" (brutal, cruel, warlike, insensitive), some think it is correct to classify Adolf Hitler as a barbarian, but that would be inaccurate.
As Merriam-Webster defines "barbarian" or "barbarous":
1 : of or relating to a land, culture, or people alien and usually believed to be inferior to another land, culture, or people.
2 : lacking refinement, learning, or artistic or literary culture.
Germany in the 1930s was anything but lacking in refinement, learning, or artistic or literary culture, and the Germans and German culture and technlogy were hardly believed to be inferior. They were ahead of the United States and most other countries at the start of WWII in science.
Call Hitler and the Nazis evil, genocidal, racist murdering miscreants. But don't confuse evil with barbarism. They key connotation you should come away with from the word barbarian is an inferiority (of culture) compared to the surrounding cultures. As such, Attila, Odoacer, Genseric, Vercingetorix, Boadicea, and Theodoric are well-known barbarian leaders who challenged the (culturally and scientifically superior) Roman Empire. One could also call Genghis Khan and the Mongol khans barbarians.