There are many reasons why Hitler hated the Jewish people.
This link will help to explain it better...
http://majorityrights.com/index.php/weblog/comments/why_hitler_hated_jews/

Keep in mind that he also hated and killed many of the Polish Catholics when he occupied Poland.
He also killed many mentally challenged people, homosexuals and anyone that didn't fit into his messed up world view.

After the WW1, the treaty of Versailles imposed very harsh sanctions to Germany
The cost of war, the sanctions and the worldwide economic depression leaded Germany in a crisis with high unemployment, strikes and instability . This troubled times were a hotbed for radical fractions, like the National Socialist Party and the Nazi ideology behind it. The Jews were seen as a scapegoat for the country's troubles-they formed a numerous minority at those times and some of them were top industrialists or bankers.
A crackpot ideology that supported racism was created. It stated that the pure German blood might become infected with Jewish blood. The Jewish were an inferior, decadent race of people. Their extermination would solve on one hand parts of the Germany's problems at that time and on the other hand, would cleanse the blood from infection. Another factor that contributed was more earthy: to disown the Jewish and take their possessions. Other factor was to exterminate them in order to reduce the cost of keeping them, for example, in supposedly unsafe ghettos.

Adolf Hitler and the Nazis killed many Jews because they were racist and they believe that German people are 'master race' and superior than others. They even have a table of races with the Aryans at the top while the Jews, Gypsies and black people at the bottom. These 'inferior' people they see as a threat to the purity and strength of the German nation. So when Nazis came to power they persecuted these people, took away right rights as human and eventually decided that they should be terminated, children, adult and old people Jewish people were not spared.