Columbus was a particularly unsavoury individual whose instinct was to sell the Taino natives he encountered as slaves, but it is difficult to imagine a scenario where the inevitable large scale contact between the Old and the New Worlds would not have resulted in calamitous declines in the native population of the Americas who had no resistance to smallpox and other Old World pathogens, no matter the nature of the intial contact. Even without Columbus, it would have been a matter of time before the Europeans found out about the Americas.

Have you read "Pastwatch: The Redemption of Christopher Columbus" by Orson Scott Card? It get's a little weirdly preachy, but the idea that Columbus really is the fulcrum about which the fate of the Americas tips, and that if you could go back and get him to think of the locals as actual people and not savages, things could work out pretty good.

If you don't mind changing history, it might be better just to go back in time and give the natives small pox vaccine. Contact was inevitable.

Hell if you are going to do that there are probably countless other things you can do to improve history.

If done even slightly wrong this would be easily detected.

Because of Columbus I and most of Latin Americans are born, most of us are a mix of European and Native Americans. If all of that population was really killed, there would be just white people in the spanish and portuguese territories.

Ignoring the rest of your comment… how would this kill you all?

My point was that the population was not exterminated in this side of America, the proof is us (Latin Americans), for example Mexico with 90% of its population being "mestizo" (european/native american) with variations of 70%/30% 50%/50% 30%/70%