Who told you he didnt know what genocide was? Columbus set off fully knowledgeable about what he was doing. His plan was to take wealth from other people by force. When he encountered the natives he remarked on how easy it would be to conquer them. He then came back with a military force and systematically exterminated the natives.
That's exploitation. Not genocide.
The goal wasn't to exterimnate them the goal was to use them. Same thing any group did to another if they had a large advantage over them.
Real genocide has to have the explicit goal of elimination as a priority, not exploitation.