Well, you got me to read the Cracked article, which I enjoyed. To summarize: we only have room for a small number of people within our "Monkeysphere" (roughly the people we can know well enough to identify as people without resorting to stereotypes) and therefore we can treat the vast majority of humanity with indifference. And of course, one way to analyse changes in moral practice over time is to examine the shifts of who morale principles apply to and who they don't.
Take for instance the Golden Rule:
And as you wish that others would do to you, do so to them.
The principle of reciprocity can be seen in all human cultures, but the key breakdown occurs with the word "others". It can be imagined, for instance, that slaves fall outside the requirement to be treated as we want to be treated because they aren't really people. We find that idea repugnant, but if we could grant that premise, the principle of reciprocity need not apply to slaves.
One area of study that may be helpful to you is Moral Foundations Theory, developed by Jonathan Haidt, which suggests these universal principles:
Care for others, protecting them from harm.
Fairness, Justice, treating others equally.
Loyalty to your group, family, nation.
Respect for tradition and legitimate authority.
Purity, avoiding disgusting things, foods, actions.
According to this theory, variations in moral systems come not from using different principles (everyone uses these five to some degree or other), but from the weight assigned to the principles. For instance, Christianity diverged from Judaism by reducing the value of 3, 4, and 5 and increasing the value of 1 and 2. Eating pork (a violation of 5) is allowed since it may allow us to treat others more equally (an affirmation of 2). See Acts 10 and this passage in particular:
And he said to them, “You yourselves know how unlawful it is for a Jew to associate with or to visit anyone of another nation, but God has shown me that I should not call any person common or unclean.” -- Acts 10:28 (ESV)
If we say that some set of principles are moral truths and if there are potential conflicts among the principles, changes in moral practice may arise not from rejecting previous moral truths, but from rejecting the weight assigned to those truths in application.
Ruben made an excellent comment below asking about some features of the Moral Foundations Theory. I should start by pointing out that it is still a work in progress as the authors are considering new principles and realigning the existing ones. There have been several suggestions that may expand the range of foundations to cover more systems of human morality.
I'd say that for the Aztecs, 3, 4, and 5 (hallmarks of sacrificial systems of worship) so outweigh 1 and 2 that sacrifice of outsiders is seen as virtuous. The victims (as I remember from history and anthropology classes) were usually captured enemies and the sacrifices symbolized the Aztec dominance over its portion of the world. In addition, excellence in battle was highly prized and being captured would be shameful, so killing captives could be seen as a sign of mercy in a twisted way.
More difficult is child sacrifice found in the ancient near east, which seems to violate family loyalty. That practice probably arose from total dominance of the "Respect" principle for particularly brutal gods. It could also arise from loyalty to the nation rather than family as it was sometimes performed by kings killing their own sons in times of national distress. (See 2 Kings 3:27.) Further, it could be that child sacrifice arose not from faulty morality, but from faulty science: if you believe that famine comes because a god demands sacrifice, you could believe that voluntarily sacrificing a small number of people could save a great many more by preventing famine.
The deeper issue is whether different weights represent different moral frameworks. I'd say that they do. Anyone finding themselves considering child sacrifice, can be confident that they've got their priorities wrong somehow. Anyone defending an action on the basis of someone else not really being human in some way, should know they've mis-weighted their moral system along the line.
Aristotle would likely suggest that weighting any morale foundation much heavier than another is itself an immoral choice. The graveyard of rejected moral systems would be filled with extreme weights like purity 1000 times more important than fairness. Moral Foundations Theory has the virtue of giving us a meta-framework that can help us understand distant cultures and analyze their moral systems. How the Aztec system arose is not an easy question, but how they went wrong morally is almost trivial if you consider the equal weighting of moral foundations an ideal system.