It could very well be argued that the Celts were far more dedicated to Law. In fact, Law had a highly religous meaning in their culture. Whereas the Romans could disregard it whenever it suited them.
In 400 BCE, when the Romans broke the treaty with the Senones (of Brennos) by assaulting their ambassadors, the Senones were shocked. They'd never even considered the possibility of deceit with regard to ambassadors. The Roman historian Livy stated that the Roman ambassadors
"broke the law of nations" (that is, broke their oath of neutrality as ambassadors).
Such laws, such as governing the security of ambassadors, were sacred to the Celts. In the resulting action, Quintus Fabius, an ambassador and a member of a powerful patrician family, killed one of the Gallic leaders (a chieftain). When the Senones realized that the sacred trust of the ambassador was broken,
they withdrew from battle to discuss the issue. (This tells you something about how flabbergasted they were).
Hence, history is well known. Brennos went for the jugular, Roma, to make them pay for it. Though it's not true that they raped and pillaged the place. That's been thrown in disrepute by modern historians. They came in, demanding a certain amount of gold. And the sum of gold was really more symbolic, to make up for the transgression of Law that the Romans had made.