Adolph Hitler wanted to kill Jews because taking that stand gave him power. When human beings work together, there is a lot of power in their combined efforts. To get many people to work together, it is very useful to make them afraid of something, for them to have a common enemy. I wish to point out that Gypsies and homosexuals also died in the Holocaust.
German did not just suddenly wake up and decide to kill Jewish people, to want to annex land from other countries.
To really understand why one group of people does anything with or to another group of people, you have to understand the history of both groups.
The Jews are an ancient ethnic group. The Romans thought the Jews were ancient enough to have earned respect when the Germans were still struggling to get out of the Iron Age. At that point in history the Jews had a country of their own.
The Romans fought with Germanic Tribes and with the Jews. Sometimes they even lost to both.
When the Emperor Hadrian drove the Jewish people out of Israel for their rebellion against Rome, they traveled to all parts of the world that they could get to. They did not integrate with the populations where they found new homes though, but instead retained their own language, customs, and religion.
For much of Europe's history, most of the European population could not read. Stories about the past were told, retold, and embellished hundred of thousands of times. Much of Europe's population also identified as Christian. (It was illegal and a capital crime not to.. meaning they'd kill you if you weren't Christian.) So having a large population within a population that thwarted the larger population's customs caused a lot of friction. One of the stories that was told commonly was that the Jews had killed Jesus. It was very conveniently forgotten that Jesus was a Jew. Most of images of Jesus that were painted in Europe depict him as a causation, which is unlikely. If he had not been Jewish, the would have had no claim at all to being the Jewish Messiah, though that's the stuff of a different answer.
So while there was historically a great deal of friction between Jewish and Christian populations in Europe, the Jews were tolerated in large part because of a small technical matter of a financial nature.
The Catholic Church was the dominate authority on religious matters from Charlemagne until Henry VIII and even after Henry on the continent. The Catholic Church forbade lending money at interest. There is a very great deal of money to be made lending money at interest. Any culture functioning above agricultural subsistence level needs a way to borrow money. Europe used the Jews as their bank.
So twenty years after WWII, while most of Germany was still in economic ruin, the Jews had money, they had physically different traits, such as darker skin, cultural differences in language and religion, so it was very easy to motivate a majority of the democratic Germany to see this nearly defenseless population as a danger, an evil.
And that's why Hitler wanted to kill the Jews, Gypsies, and homosexuals.