First off the definition of holocaust is a mass murdering of people esp. in nuclear warfare.
One of Adolf Hitler's motivations for starting the holocaust was that he wanted a pure race to live in Germany. He didn't want any foreigners and had many women treated unfairly so they could only focus on treating their baby with the highest of standards. The "master race" that he wanted to create would have been made up out of the most athletic, smartest people that he could get his hands on. You have to admit, with a race made purely out of these kinds of people would be a very strong German community.
There was a strong anti-Semitic tradition in Europe which evoked the Nazis' to rise to power and essentially slaughter all the Jewish people in Europe but mostly in Germany. This tradition was just carried on by generations because they thought Jewish people were against them and they just basically didn't like them.
One of the theories for Adolf Hitler's motivations if that he was provoked by his Jewish father, Alois Hitler, who often beat him and put him down. Alois didn't believe in in his child and only wanted Adolf to follow in his footsteps. This led Hitler to get angry and him and his dad had raging fights of fury. But we are not completely sure that he was really provoked to kill millions of people from this, although like I said it may be a factor.