I wonder what would be of current Colombia without the Spanish people that conquered it.
As for it being mostly monolingual under the imagined circumstance, I can't say, but it's possible Gran Colombia (current-day Colombia, Venezuela, Ecuador, Peru, Panama, etc.) would have naturally formed and, under it, a monolingual pidgin.
Aztecs - Mexico Maya - Guatemala and neighbouring countries.
Here's some reading: https://en.wikipedia.org/wiki/Black_Legend
As someone that loves to study history, read old mediaeval documents, I know pretty well what our ancestors did.
I get it, condemning their actions gives you the higher moral ground, you feel like a mensch and somehow detach you from whatever shared responsibility you think you shoulder.
IMHO, we can't judge old-day actions with current-day ethics. The same way we don't generally allow ex post facto law, we should be careful to deliver ex post facto moral judgement. If there's a place for moral relativism, it is probably when comparing different times in history.
That is not to say I excuse everything that was done during the colonial times —there are plenty of recollections of senseless atrocities— but there's definitely a tendency to look at all this in a reductionistic and emotional way that I don't think is fair.
The spaniards have been better prepared for war and overcame the local population in bloody conquest — this is a historical fact that no one will argue with.
How, from this fact, do people come to an assumption that local population had higher moral values than invading spaniards?
Have you completely no idea what pre-columbian era indian cultures were like? I mean, not today's "New Age" versions that have been adopted for Californian yappies, but actual kingdoms, chiefdoms and empires that resided there several hundred years ago.
The spaniards alone were a skeleton crew, not enough to conquest anything that big, despite having better technology.