Have you ever met a holocaust surviver; seen the tattoo in faded blue ink on their arms? I have known just a few. Most are extremely old now. I doubt my own children will have the opportunity to meet one. My brother-in-law's father was a POW housed at Dachau. Admittedly, he wasn't Jewish, but he did see what went on at that camp.
Have you ever been to any of the camps? I have not and probably never will. It is too disturbing for me.
The evidence for the holocaust is simply overwhelming. Even those who would LIKE to deny it (ie, Germany) have made it illegal to do so. The fear is that "those who forget history are doomed to repeat it."
There have always been conspiracy theorists and I suspect there always will be. They pop up in every catastrophy. Why, did you know that it was actually the Jews and the US government that brought down the Twin Towers?......
These people come up with their lunatic theories for various reasons. They are not lying for the most part because the really do believe their own bull****. It is sad really.
If you really want to know what happened, go visit the camps. Visit a few Holocaust museums. Sift through actual data and records written by the Nazis themselves (say what you like but they were meticulous record keepers). Maybe even talk to a few survivors if you can find any (and if you can get them to talk of their horrific experiences).