Do you read much history? I only ask because Columbus was only one in a very long, long line.
We talk about how great Genghis Khan and his empire was, despite the fact he and his Mongols slaughtered countless people and completely wiped out a culture or two. Yet Khan is considered to be a great man because of the empire he built.
It's been no different throughout history, whether it was the Egyptians, Persians, Greeks, Romans, Babylonians, etc. But the moment we talk of European expansion and conquest and empire, suddenly we look at it through modern day eyes with a 21st century mindset and judge it accordingly.
You can't reasonably judge Columbus when you are 500+ years removed from the event. Different times, different attitudes, different mores.
Having said all that, I have no idea why Columbus gets credit for discovering the Americas when it was Bronze Age Britons( Fishermen from England's south-west most likely) that were the first Europeans to "discover" the Americas some 400-600 years before the Vikings, who were about 500 years ahead of Columbus.