Barbarus was borrowed from the Greeks, for whom it originally meant simply "foreign", because foreign speech sounded like rhubarb rhubarb or because foreigners seemed to stutter. It was then argued that if to be barbaric was to be foreign, and foreigners, as everyone knew, were cruel, then to be cruel was to be barbaric, whatever the country of origin. Cicero used it in either sense.
The real trouble with "cruel and barbaric" is that barbaric is the wrong word, or ought to be. Barbarus was the right one for a bloody foreigner. But Latin also had barbaricus, which meant not so much uncouth as exotic - foreign certainly, but not necessarily bloody; we took it over in the 15th century, and, being even more xenophobic than the Romans, made it do for everything they had meant by barbarus - a gabbler, a rogue, or more likely both. Milton used the word in its proper classical sense when he wrote splendidly of "where the gorgeous East with richest hand / Showers on her kings barbaric pearl and gold", though of course there were puritanical undertones of disapproval here; and the 19th-century historian George Grote was being rude to no one with his reference to "barbaric-speaking nations".
It's a shame we ditched the distinction between barbarous and barbaric, not so much because we've lost a nice Miltonic word, but because barbarous would be far more telling, yet most people prefer barbaric.