Does it have to be one or the other?  He was gerat in that he had the courage to explore unknown places (educated people of the time did *not* believe they would fall off the edge of the world), and some of his crew were probably people who would otherwise been killed by superstitious oafs, so he was likely responsible for saving lives.  On the other hand, he started the takeover of the "New World" and spread diseases that killed large numbers of the native population.