This is probably because they were in fact murdering, torturing savages given to human sacrifice. Why this is thought especially reprehensible is a mystery. The Romans sacrificed large numbers... so that the public could enjoy watching them die, crucified large numbers, and poured molten lead down the throats of criminals. In the European witch hunts, sort of 1450-1750, some 500,000 were killed depending on whose numbers you accept... The Tudors hanged criminals, cut them down still conscious, opened their abdomens and removed their bowels while still alive... . And of course everybody and his dog put entire cities to the sword, from Joshua to Hiroshima. Despite their best efforts the Maya could not keep up with the moderns.
Why was Mayan human sacrifice 'worse' than the bloodbaths of Western Civilizations? Romans shed blood for entertainment or punishment. Now, killing people for entertainment is ugly and grisly, but it is what it is. Mass spectacle. Romans didn't pretend otherwise, as if it could be anything more than bloody fun. People like to see violence. Lots of people love violent movies and violent video-games. People love to see animal programs where predators hunt down prey. Americans are addicted to violent football and fighting sports. Romans did it for real, with bodies piling up in earnest. But Romans never claimed there was any moral or spiritual dimension to gladiator sports. As for practices like the crucifixion, they were meant as punishment. A bit extreme perhaps but still a tool of 'justice' and power, nothing more and nothing less. Draconian but meant to send a strong message to make people comply with the Order. At the very least, the innocent weren't randomly summoned to be crucified. (It's also worth mentioning that gladiatorial sports could, in some ways, be a crude form of justice, at least according to the movie GLADIATOR. Certain criminals were forced into it. Also, conquered men of other tribes who were spared death were given a second chance to prove their mettle in the arena.)
As for European witch-hunts, while they could be ugly and violent, the point was to hunt down demonic forces and root out evil. Europeans believed that certain individuals were possessed of evil spirits or practiced witchcraft in service to Satan. So, they believed 'witches' must be exposed and punished in the name of God. Witch-hunts were NOT carried out to honor Devil but to ward him off.
As for the Tudors, they certainly had an ugly way to handling criminals, but it was still a form of justice, punishment for bad elements.
In any case, one could put forth moral and/or rational arguments against the practices of the Romans, Christians, and Tudors. One could argue that gladiatorial sports, while fun and exciting, were cruel and ugly. One could argue that hunting witches, though in the name of God against Satan, was excessive and unforgiving(against the preaching of Jesus). And one could argue that the Tudors were a bit too outlandish in their treatment of criminals. Even if we agree that earlier European civilizations acted in ugly, gross, and excessive ways, we can still understand the motivations behind them on the human and even moral level. Romans favored thrill over good-will in their addiction to blood spectacles. And Romans, Christians, and Tudors were overly zealous in meting out punishment or rooting out Evil.
But what is to make of the Mayan human sacrifice? It wasn't for sports or entertainment, which would have been ugly but understandable as spectacle & entertainment. It wasn't to root out evil(as in the witch-hunts common in Medieval Europe). It wasn't to punish the criminal elements as most who were killed hadn't committed any crime.
It was a ritual of sacrificing innocents to amoral or even immoral gods. The very cosmology of the Mayans was morbid and psychotic. It believed that the universe is ruled by cruel, uncaring, and sadistic gods who demand sacrifice of the innocent. Against such cosmological psychopathy, no moral or rational argument could be put forth. The culture was demonic and demented at its very core. Christians misused the teachings of the just God. Mayans correctly served their immoral gods who demanded the slaughter of innocents.
And that may have been why no South American civilization achieved traction and longterm viability. They could make art, do math, pursue astronomy, and build temples. But they failed to conceive of a deep and resilient theory or vision of justice.
Greeks and Romans, even as pagans, believed in gods possessed of the power of reason and justice, though far from perfect themselves. Apollo and Athena upheld reason, order, and intelligence. Zeus was not without sentiment and, when not indulging his whims with the ladies, tried to be fair-and-balanced with other gods and humans.
And even though Jews and Christians stained their hands with blood time and time again, their conception of God was one of power AND justice. In contrast, the spiritual conceptions of South American civilizations were dead in soul. While alive in the arts, architecture, and rudiments of 'science' and math, they failed to bridge the gap between cosmology and rationality and morality. Their gods were all about the whims of power.
Civilization after civilization, from Mayans to Aztecs to Incas, they fell into the trap of ritual human sacrifice to appease the appetites of amoral or even immoral gods. When a people believe that even the gods lack souls, they are bound to fall no matter their achievement in monuments and math.
As for modern wars and their massacre of millions, war is hell. Always has been and always will be.