Why did most of the native Americans die when they first came into contact with the europeans?

Answer 1

As with all cultural competition over resources, the ones with the most effective technology wins out. As the population of the world grows, all kinds of warfare are used in this endeavor, regardless of cultural background.
What we would call biological warfare today was used in the Middle Ages when they catapulted bodies with black plague over castle walls - The "White Man" gave the "Indians" blankets with small pox and the like. To which they had no resistance.
The "White man" used modern rifles to shoot millions of buffalo simply to deprive them of their food, clothing & shelter, and deplete their military defenses.
Many "Indians" committed as many atrocities to be sure, but they'd been doing the same amongst competing tribes anyway in any event, but only with bows, arrows & knives.
In this day and age, Red China has committed decades of economic and similar bio-, political, and warfare in a prelude to overt military action, all exploiting their slave labor in factories to produce cheap and poisonous goods to import to the US people.
The Indians failed to adapt for spiritual reasons. But, who knows? When push come to shove - Maybe "Planet X" and world-wide electromagnetic pulse happens soon? As supposed to happen every 3,600 years?
Will the Indigenous then live, perhaps, in harmony with "Nature," again?
And the Indigenous will simply be a hegemony of those who survive.
So go learn the art of survival, I urge you.