alternate history

Some other person would've credited for what he was doing. Everybody was sending exploratory vessels out to do stuff. If Spain didn't some other European nation would have tried it eventually in the time period because they were all getting technology and discovery obsessed.

Arguably certain island nations that ended up under his governership would have had someone slightly less brutal but I can't imagine it would have been much better for the indigenous peoples. Even someone more on the level than Columbus was still going to try to subjugate the indigenous people to produce personal profits for themselves and increase their social standing in rare circles of power they cared about back home.

All in all Columbuses Contemporaries thought he was something of a charismatic idiot. They already knew the world was round and a lot of them had much more accurate calculations than Columbus which is why the first three countries refused to bankroll him. Some advised that if there was nothing but water on the other side of the earth you'd just all run out of viable provisions 1/3 of the way no matter how much you packed on the boat as spoilage, tonnage and the sheer amount of viable fresh water required created hard limits to how long you could be at sea particularly if anything went at all wrong causing unexpected delays.

But someone was bound to try exploring at some point. They probably wouldn't have "discovered" things in the same order... But they would have got there.