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.