There's a lot of sloppy thinking in this article. The Romans were more civilized than the Celts and Germans, but it is not clear to me that the Romans were any better than those barbarians. The Persians may have been more civilized than the Athenians. Derbyshire is trying to argue against terrorism while avoiding reference to right and wrong. But that is impossible. The civilized state in the historical perspective has been the sedentary, rich society, while the barbarians have the nomadic and poor society. Thus, the Philistines were probably more civilized than the ancient Israelites. So it is not always true that the civilized are to be supported over the barbarian; what matters is who is on the side of good and who on the side of evil.
You are now required to define evil.
"The Romans were more civilized than the Celts and Germans, but it is not clear to me that the Romans were any better than those barbarians."
From the article: "There you have the difference between civilization and barbarism. If you can't see it, I can't help you: You are morally blind."
The whole theme of the essay is that the civilization/barbarism distinction can be a slippery one. Civilized societies can be at once better and worse than barbaric ones, but it's only the civilized who have the relative safety and leisure to contemplate good and evil.
Civilization vs. barbarism is one of those oppositions like nature vs. nurture that will never be exhausted. When John Milton was at University, the question of whether civilization is better than barbarism was one of the topics routinely assigned for the the students to debate. (Milton originally wanted to argue for barbarism, but after he thought about it, asked to argue for civilization.)