Would it have been the moral thing to do to turn around and leave the Indians alone, instead of taking their land and using it to build an advancing civilization that can support a population of about 100 times as many people, who think they are living more pleasurable and interesting lives, and hardly ever cut out their neighbors' hearts on the tops of temples to the sun god?

Dude, false dichotomy. What if the colonists had just colonized America without being such total dicks about it?

I bet there are plausible scenarios leading from such a policy that would've led to about the same level of awesomeness on the American continent that we see today, or possibly more awesomeness.

Edit: I see this has already been addressed below. These pre-threading conversations are disorienting.