Well, it is one of the things that was drilled into us all life long that there is a progress in history from barbaric behavior to "nice" (or, properly, ethical, compassionate) behavior.

But beyond that there is indeed a process of increasing cooperation. There is usually peace inside nations and empires, i.e. they don't allow their constitutent tribes, if they still exist, to fight each other. While this increasing cooperation is largely about banding together to fight someone else, still it creates a certain progress in morals. Thucydides wrote that the ancestors of Greeks did not know the idea of peace. They just pirated on each other all the time. Peace was invented through military alliance: teamed up against some other polis and then realized it is not nice to pirate on your ally. So through banding up to fight someone else, internal cooperation and peace is acheived. And you cannot have an advanced culture without it. There is no economic progress with a deep division of labor, meaning trade, meaning peace within the empire. And this success of cooperation permeates then religion and philosophy and people's values.

Sorry, I think I dumped a bit of an unedited train of thought on you. I'll try to organize better. So advanced means rich, rich means division of labor, div. of labor means trade, trade means peace inside the empire, means a value system that values cooperation and peace increasingly, then the whole thing gets justified by philosophy, art and religion which ingraines the values even more, and then they should be shocked by bloody spectacles.