Why were the Native Americans so far behind in technology?
No horses. Without horses there were no wheels or long distance transportation. Without long distance transportation there wasn't the creation of a trade network or the need for a common language over long distances. Most Native Americans remained where they were whereas the Europeans traveled over vast distances, traded, started wars of conquest and so on.
Why are only Europeans blamed for the slave trade and colonialism?
Because they were the core participants and perpetrators. While it is true Europeans were tapping into a pre-existent slavery market in west Africa, the nature was entirely different. Slaves in west Africa were often not taken far from their place of origin where they'd continue to practice the same culture and religion. Unlike the largely agricultural intensive labor Europeans forced their slaves to do African slaves in West Africa often did small scale homestead work alongside their owners. Slaves were given a degree of freedom and many opportunities to leave. While this is by no means benign these small cases of slavery do not compare to the large scale mass enslavememt and forced labor carried out by Europeans where slave ships with the worst conditions imaginable hauled slaves thousands of miles away in bulk. In many European Colonies such as Haiti, the death rate was around 90%.Famous diagram of European Slave ShipThe triangle tradeNative Americans were not just killed off by small pox, they were used by the Spanish in massive forced labor projects and saw themselves being pushed off of their land by force.And while it is true Natives engaged in violent battles against one another before Europeans, it was often individuals or small groups being killed back and forth for reconciliation. Europeans specifically encouraged Native rivalries and wars, pushing natives (now empowered by guns and later horses from the fur and skin trade with Europeans) farther west where they'd push subsequent tribes west causing a chain reaction analogous to the Hunnic Empires effect on Germanic Tribes of Eastern Europe.When people bring up these things it's not that they're trying to make Europeans and Westerners feel guilty, they're being historically honest and showing the horrors and atrocities of colonialism so that those who suffer are not forgotten and those who committed the crimes are acknowledged for who they are.And while yes, Europeans were the ones to abolish slavery, just like how all Europeans can't be blamed as a monolithic entity for specific actions made by individuals (and individuals working a single collective) in the past, neither can they take credit for actions in the past made by separate people. Also in the 1830s when Europeans ruled the world who else could have abolished slavery?
Why were the Aztecs, Mayas and Incas so far ahead of the natives of the African civilization?
In many ways, they weren’t. The Ethiopians could access Asian and European technology, and were masters of horse breeding. The Bantu peoples invented iron in the continent and spread it across the entire continent. The city of Timbuktu may not have been as populous as Tenochtitlan, and even Mali never reached the size of Tawantinsuyu, but they were every bit as culturally sophisticated. North Africa adopted Arabic and Roman cultures and integrated them with local systems. Others like the Yoruba kingdoms developed their own city states. The Swalihi coast had frequent trade with India.Really, when it comes to Africa and the Americas, it’s surprising the Americans look so good despite the far superior technology, higher literacy, and cultural diversity of Africa.