I think people remember Columbus because the good outweighs the bad. Who cares about the natives anyways >.> People probably remember Columbus as 90% good and 10% bad, or maybe 99% good and 1% bad. In China, the Communist Party evaluated Mao as being 70% good and 30% bad So it really depends on the perspective, cuz I doubt many people in this community could agree with that. I don't think it's right to just focus on what bad somebody does - because I'd imagine that attitudes also tends to only focus on what good somebody else does.
If we look at atrocities for what they were, then there's no way to ignore how terrifying they were. But we often look at atrocities for what they are. Sometimes things, rightfully or not, just aren't relevant nowadays. Even the sting of the Holocaust can be dulled with time if people stop caring.
I'm not so sure about offended or upset. I can feel offended, or get a visceral response, looking at images of the Holocaust for example, but when thinking about it in general - I don't know what's to be offended or upset about. To be offended or upset is to have that instinctual response to a threat, and whether we react to something as a threat or not really depends on our person.