I don't find the word noble appropriate for most circumstances. Aztecs didn't just throw people into volcanoes, they left neighboring territory unconquered to have wars to take prisoners, who they would capture for human sacrifice to their animistic gods. With the prisoners, they would slice open their chests and pull out a beating heart, which they would hold up to the cheering crowd who hoped to get a drop of blood land on their clothing. These "noble" people would eventually suffer from great rebellion and unrest, but the Europeans came just in time to prevent a probable series of wars. Contacts between the Old and New World is inevitable, so if the Europeans came any later there would probably be great disorder along with several more deaths from the carried diseases.

Judging by the patterns of other civilizations, the Native Americans would still develop an industrial workforce and steer away from most concerns about their religious beliefs as they discover astronomy and other scientific breakthroughs that were previously known by the Old World. It's not the Europeans' fault for the end of the Native American lifestyle, because it was just a matter of time before changes would occur.