Answer

I wrote about this in the third book of the trilogy. In brief, the gentiles of old were people whose human norms were very degraded, almost like animals. Those are the gentiles that the Sages had before their eyes. They did not accept upon themselves the seven Noahide commandments, which include murder, adultery, idolatry, and the rest of those “virtues.” Therefore the attitude toward them was accordingly—not only as a sanction, but also in order to educate Jews not to be like that. If there is a group that behaves toward you like animals, there is no reason not to treat them similarly. People at that human level certainly were not careful about verbal mistreatment toward their peers or toward us. So why should we be careful about that toward them? The same is true regarding returning lost property. If there is a group that keeps all lost items for itself, why shouldn’t I keep their lost items for myself? And so on.
Nowadays the situation is completely different, and therefore the law also changes. The Meiri already wrote this in the 13th–14th century, and it is even more true nowadays.
One should know that already in the days of the Sages, a resident alien (a gentile who accepts upon himself the seven Noahide commandments) merited treatment similar to that given to a Jew. Today most gentiles are like that.
And beyond all that, there are basic norms that obligate us toward every human being (what are called human rights), and there are norms that obligate us toward those close to us (citizens of one’s country, family members, and the like—what are called civil rights). I am under no obligation to provide education to every person in the world, but I am forbidden to murder or harm any person in the world. Therefore some preference for our own people over gentiles is logical and reasonable, just as you would not invest all your money to save a stranger from dying of an illness, but for your son, your wife, or your parents you certainly would do so. But, as stated, that is only with respect to civil rights. Human rights belong equally to everyone.