The Greek researcher and storyteller Herodotus of Halicarnassus (fifth century BCE) was the world's first historian. In The Histories, he describes the expansion of the Achaemenid empire under its kings Cyrus the Great, Cambyses and Darius I the Great, culminating in king Xerxes' expedition in 480 BCE against the Greeks, which met with disaster in the naval engagement at Salamis and the battles at Plataea and Mycale. Herodotus' remarkable book also contains excellent ethnographic descriptions of the peoples that the Persians have conquered, fairy tales, gossip, legends, and a very humanitarian morale. (A summary with some historical comments can be found here.)
Herodotus rarely condemns foreign behavior, even things he must have considered very shocking. When he has told that Persian king Cambyses has attacked the Apis Bull, which is sacred to the Egyptians, he unexpectedly offers a biting comment that is, essentially, a summary of Herodotus' views on foreign civilizations.
The translation was made by Aubrey de Sélincourt.
Herodotus' Comment on Cambyses' Madness
[3.38] In view of all this, I have no doubt that Cambyses was completely out of his mind; it is the only possible explanation of his assault upon, and mockery of, everything which ancient law and custom have made sacred in Egypt.
If anyone, no matter who, were given the opportunity of choosing from amongst all the nations in the world the set of beliefs which he thought best, he would inevitably - after careful considerations of their relative merits - choose that of his own country. Everyone without exception believes his own native customs, and the religion he was brought up in, to be the best; and that being so, it is unlikely that anyone but a madman would mock at such things. There is abundant evidence that this is the universal feeling about the ancient customs of one's country.
One might recall, for example, an anecdote of Darius. When he was king of Persia, he summoned the Greeks who happened to be present at his court, and asked them what they would take to eat the dead bodies of their fathers. They replied that they would not do it for any money in the world. Later, in the presence of the Greeks, and through an interpreter, so that they could understand what was said, he asked some Indians of the tribe called Callatiae, who do in fact eat their parents' dead bodies, what they would take to burn them. They uttered a cry of horror and forbade him to mention such a dreadful thing. One can see by this what custom can do.