Why is it bad and some cases illegal to question the "holocaust"?

It just seems kinda fishy that when anyone says I don't believe the holocaust happened and people get offended and feel as if it is an ault. This is just ridiculous for anyone to do this because if I was to go around and say slavery never happened they would just show me the evidence and laugh at how stupid I was. This is not the case at all with the holocaust. In fact it was illegal for anyone who lived in the area where the holocaust happened to say it didn't. This to me smells very fishy because these people are probably the most creditable sources to possibly debunk the story so to make it illegal for them to speak out is like throwing chains on any possible counter to that story. Not only that but Benjamin Freedman called out in WW1 that the zionists of Germany and masons of Britain had a deal where the zionists would undermine Germany so that Britain can be the new superpower in exchange the zionists get land in Palestine. Must be a coincidence that after WW2 Israel sprang up from the ground. Anyway if asking a question is an ault then there must be a reason behind it because the truth needs no curtain or law to hinder anyone from speaking out. As for anyone trying to bring up evidence. Eye witness accounts can be flawed in the idea people can lie so if you could please exclude that in the evidence column then we can really begin to piece the puzzle.