I haven't dug too deeply into the matter, so I can't say I know all there is to know about the Holocaust, but I do know the following:
1. Holocaust deniers are almost never able to have merit on their position without SOME form of hatred and qualms for the Jews. Their stance is often heavily fueled by this (i.E. Jews are liars and control everything, so could very well have made up lies to get what they wanted)
2. A substantial amount of Holocaust deniers have passed themselves off as credible experts in relevant fields, when actually they were not.
3. Making a claim before it is substantiated by facts, then using isolated and circumstantial facts to justify these claims.
4. Many of their claims are self-contradicting- one claim is that there weren't that many Jews in Europe to make it possible, another is that the gas chambers were incapable of exterminating this many people in the short period of time, and yet another is that gas chambers weren't used. The first claim is ultimately dismissive of the possibility of such a high number of casualties, while the latter two are like 'they just didn't have the appropriate mechanism to get it done'
The Holocaust is a proven historical event. I think it's sad that people want to deny that it ever occurred. Look at all of the Holocaust survivors; millions of people who all had similar experiences, and went through many hardships. It does not make sense to believe that millions of people are lying about the fact.
That being said, I think Holocaust deniers are stupid. Even moreso, are the people who attribute those people with any sense of good or evil. Look, idiots, "denial" by its very definition simply means you don't believe something happened. The only time you can attribute good and evil to someone in regards to the Holocaust is if they take support it or condemn it. Holocaust denial is the dumbest phenomena. The Holocaust did indeed happen. Whether you support this or not, is another story.