In fact, the word barbarian did not start off as a term with a negative connotation. It was indeed a onomatopoetic word of the sounds non-Greeks made. Barbaroi was just a collective name for all non-Greeks, like we would use a term like foreigners or aliens. It might be argued that Greeks thought of barbaroi as filthy and strange, but certainly not as much as they would in later times.
During the Persian wars, the contrasts between free Greeks and servile Persians were emphasized. The consequence was that the word barbaros also shifted somewhat in meaning. By the fourth century before Christ, barbaros stood for all natural enemies of the Greek, who would be cowardly, rude, cruel, unreliable, argumentative and greedy. Not like the Greeks, of course (to cite Cletus the Foetus).