This is, of course, a matter of personal opinion. Your answer will depend on your definition of a hero and on how much bad a person can do and still be considered a hero. My own view is that Columbus is not a hero.
To me, a hero is someone who knowingly does something that benefits humankind and who does so for reasons other than personal gain. I do not feel that Columbus meets any of these criteria.
We can certainly argue that “discovering” America, on balance, helped humankind. It definitely harmed the natives of the Americas, but it greatly benefitted the Europeans who came to the Americas as colonists. The creation of the United States (which came about because of the discovery of the Americas) has surely helped the world more than it has hurt it. Thus, Columbus did help humankind in the long run.
However, he did not do this knowingly. Columbus never believed that he had discovered a new continent. He died believing that he had reached Asia. He did not mean to discover a new continent and never believed that he had done so. That means he did not knowingly do good.
Finally, Columbus was trying to help himself, not humankind. Columbus worked hard to finally gain the funding for his expedition. However, he did so because he wanted wealth and power. He wanted the wealth that would come from reaching Asia and bringing back spices. He wanted the power that would come from being granted the right to rule “new” lands that he found. He was out for personal gain, not to help humanity.
We can also add the fact that Columbus clearly mistreated the people of the lands that he “discovered.” Heroes do not have to be perfect people. However, it is hard for me to think Columbus is a hero when he came to the Americas, saw the natives, and immediately thought of enslaving them and forcing them to work for him.
Columbus undoubtedly accomplished something that no one had done before, something that has been good for humanity as a whole in the long run. However, he did not knowingly do this and he did it solely for personal gain. It is therefore hard to argue that he was a hero.