Why might celebrating Columbus Day be offensive to Native Americans?

Yes, Columbus was a terrible human being and the fact that he is still celebrated should offend anyone living in the United States.

It turns out that most of what people know about Columbus is a myth.

For instance, it is obvious that Columbus underestimated the size of the earth and overestimated the size of Asia (otherwise he wouldn't have gone on his trip at all). But contrary to what many people believe, learned men of his time not only also knew that the earth was round, they actually had better estimates of both the size of the earth and of Asia, which is the real reason why his project was rejected by several monarchs before Isabella.

Therefore Columbus discovered America because he was ignorant, not because he was a visionary.