I think that there are many factors that caused the Holocaust to happen. The first and biggest factor is Hitler of course. I don't think it was Hitler's fault solely, but I think that he must of had someone close to him that was Jewish. Either this person hurt him or someone he knew, or Hitler just didn't like that person. If that wasn't the case, then I think Hitler must of had relatives or friends when he was young that passed down antisemitic views down to him. Thus, one or both of those situations caused Hitler to write what he did in Mein Kampf.
The second factor that I believe led to the Holocaust was the harsh treatment of Germany in the Treaty of Versailles. If this Treaty didn't completely embarrass Germany, the people of this nation would not have been in a place where they were so willing to accept any new views to get them out of it. This is why Mein Kampf became so popular and led to Hitler being appointed as Chancellor. When Hitler was appointed as Chancellor, his party followers grew stronger and became more influential. This led to Hitler becoming dictator and creating the Third Reich. I believe if just one of these events didn't happen, neither would have the Holocaust. This period of time had a ripple effect where one event built on another until one of the worst things in history happened.