In terms of a perversity of soul, Holocaust denial says something far darker about a person than young earth creationism.
Young earth creationism is incorrect about the age of the earth and the fixity of species, but it is not an inhumane way of perceiving the world. It is not psychologically souless.
Contrast this with Holocaust denial. You have to be someone who trolls around the darkest parts of the internet to be a Holocaust denier, visiting (for example) Neo-Nazi and Free-Masonry/Illuminati style conspiracy websites, or receiving, via the mail, crank literature written by Antisemites.
No fundamentally decent, humane, or rational person can be a Holocaust denier. There is no warrant (intellectually) for denying the basic facts of the Holocaust (approximately six million Jews dead; the use of gas at the death camps; the genocidal intent of the Nazi government)—and yet there are people who go out of their way to do so.
There is no reason—none whatsoever—for giving the crank literature on Holocaust denial greater weight than the measured and sane conclusions of mainstream historians working at the world’s best universities.
And yet there are people who do.
In short, there are people HIGHLY MOTIVATED to deny to a particular people (the Jews) a pivotal moment of their history.
Just as we would say that there is something psychologically perverse and racist in a person who denied or downplayed the extent and severity of the slave trade during the 18th and 19th century, so we must not put on blinders concerning Holocaust denial, and simply pretend that it is just another intellectual position that a reasonable person, looking dispassionately at the evidence, might simply arrive at.
Echoing the old Frank Sinatra song, Holocaust denial and Antisemitism go together “like a horse and carriage.”
Let’s not pretend that Holocaust denial is something that it’s not. It’s not a dispassion. It’s a prejudice. And it comes from the very darkest parts of the human psyche.