If there was no WWI would there still have been a Holocaust? What if Germany's economy never really suffered, then there would be no reason to blame a certain group. So maybe the Holocaust would not of happened. Or if Hitler never became Chancellor of Germany or what if he never even existed. Maybe if another powerful figure was brave enough to take on Germany's problems, he could of turned Germany's economic situation around and brought stability back into the country without hatred.
Traveling further back in history, what if the past German culture was not as anti-semetic. And the Jewish people were more accepted through out the world. This factor easily could of changed the events.