Why do some white supremacists deny the Holocaust?

Same reason some Christians still tell you the earth is 6,000 years old. The very same reason you will have people who still believe Hilary emails were somehow breaking some laws. You get some people that will believe what they want regardless of what others say and what happens around them. To them the Holocaust will be like global warming to a Republican. They won't believe it happened as it means you have to admit you were wrong.

David Duke is their hero!

Its because they have a hidden agenda.

Because it puts people against them and stops them doing it again

the six million figure is a lie. zionist propagandists were using that figure all the way back at the end of the nineteenth century, claiming the russians were out to get them. they always vilify and falsely accuse their intended victims, as in the soon to follow jewish led and financed bolshevik revolution. zionist leaders sacrificed some of their, "lesser brethren" for the cause of getting a homeland in palestine supposedly for their protection, but the real reason was just the opposite.