So first off you can't place modern definitions or connotations on words from previous eras. We have to examine it within the context of the time period you're referring to. So barbarian comes from the Greek barbaros, which means 'babbler" (Im pretty sure) Its how Greeks understood or heard the language of other cultural groups, it sounded like gibberish to them or "bar bar". But from what I remember it is onomatopoeia which is a word that resembles the sound it spells, like in English sizzle ding thump.

The Greeks would use barbarian or barbaros to refer to any other nation i.e. Egypt, Persia etc or basically non-Greek speaking nations. Rome had transformed the definition of barbarian which still referred to foreign nations/cultures but of those who lacked Greek or Roman traditions like the Gothic tribes. After all the Romans were barbarians to Greeks right? It is later scholars who would further transform to term to imply those who attacked civilized nations. We also have to take into account how much the Gothic and Germanic tribes were hated by their contemporaries and later scholars, even in the late classical and early "Dark Ages". I believe it was Chirs Wickham in The Inheritance of Rome: Illuminating the Dark Ages said that the Germanic tribe the Vandals were hated so much and seen in a negative light its where our current word vandal gets it's meaning from.