It would seem incredible that anyone would not believe in something so well-documented as the holocaust. And we might think that anyone who denies it is doing so purely out of hatred or spite (and I suppose that might be so for some). I have come to realize that there are some people in this world whose minds simply do not work in the way the majority of us consider “normal”. Reality for them is not what reality is for the rest of us and the way facts and reason are used to generate reality for them is not the way it is for most of us.
Lest we get complacent, remember how easily normal people have gotten turned to madness by propaganda, war and misguided religious zeal.