I would say that any textbook should portray Christopher Columbus in a way that does not deny either his good or his bad qualities. He should not be made a hero, but he should not be seen as a complete villain either.
Columbus clearly had admirable qualities. Notably, he had a great deal of drive and ambition. He was willing to work very hard to get the chance to prove his theory that he could reach Asia by sailing to the west. He must have been very driven and very brave to try something that had never been done before.
On the other hand, we cannot give him too much credit for what he did. He only discovered America accidentally. He never admitted that he had not reached Asia. Thus, he did not purposely accomplish the thing for which he is most famous. This is not to his credit.
Columbus clearly did some bad things as well. Most notably, he was willing to mistreat the natives of the lands he “discovered.” He enslaved many of them. He punished Indians who failed to bring him the amounts of gold that he required. These were clearly bad things to do.
However, we should also not condemn Columbus as harshly as some people do. Some people say that he perpetrated genocide against the Native Americans. This is unjust in my view. Most of the deaths among the Native Americans were caused by disease and not by conscious actions on the part of Columbus. It is not fair to blame him for those deaths.
Thus, any textbook should be balanced in its portrayal of Columbus, saying that he had both good and bad aspects, but not exaggerating either of those types of qualities.
The best way to portray any historical figure, especially one involved in controversy, is to simply report the events of their lives and leave your personal feelings at the door.
This not only encourages Socratic discussion, which is the best way to handle historical studies in general, but it helps to avoid the students' fear of having the "right" answer, of which history can rarely boil down to.
More than anything, we needn't judge history from a modern perspective, as this is not only unhelpful, it sets a bad precedent for how our own society will be examined in future centuries.