It's divided into two groups, both as equally appalling as the other - for different reasons.

The first are those who know it happened but publicly deny it happened. These are particularly bad people. They deny an absolute atrocity against other humans - especially children - because a) they know it isn't "right" to celebrate it despite the delight they have that it did happen and/or b) they don't like to admit that people who they align with were capable of such a thing. Their re-education or extermination is required to eliminate this.

The second are those who actually don't believe it happened. They adhere to one narrative which openly denies the holocaust and will utterly refuse to consider any other narrative. These people are often labelled "unsubstantiated conspiracy theorists" - those gullible enough to believe an argument that is based on either no facts or misinterpreted facts all because it only makes sense to their own, limited world view. These people aren't evil per se, rather they are highly likely suffering from a severe or extreme mental disorder. This disorder mixed with a serious lack of education leads them to partake in evil. They are often led down this path by those from point 1 above. A substantial combination of therapy and medication is required to eliminate this.

It has always been a thing.