Anit-Semitism was actually a common sentiment in Europe and most of the world at the time. Germany was far from being the worst of culprits in persecution of the Jews. Hitler simply acted on his belief when he had the opportunity and power to do so, as did several other leaders, all supported by their countries and people (apart from the Jews that lived there, of course).
As to why the idea was so popular, it just was and always has been.
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The only groups singled out for extermination rather than enslavement were the Jews and the Romanies ('gypsies'). Please see related question.
they did not see it as their job to step in.
They were taken to extermination camps. Please see related question.
The Nazis hated the Jews. Jews were put into ghettos or already lived there. See related links below to tell the story. ____ They showed off Theresienstadt to the Danish Red Cross as a model ghetto.
From 1933 to 1938 the Nazis made it increasingly difficult for Jews in Germany to earn a living. They were also subject to petty and vindictive restrictions. Then, in November 1938, the government organized the Night of the Broken Glass, after which there was a stampede to get out of the country. Please see related question.
No. The primary reason that the Nazis killed the Jews was that viewed the Jews as vermin to be eradicated in much the same way as most people see mice as vermin to be eradicated. (Which is, of course, a virulent and extremist racist view.) However, Nazis were not above pilfering wealthy items from Jews that they were going to kill, especially paintings, jewelry, or large bank accounts.
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In 1918 - none. The year you are probably looking for is 1933, when the Nazis came to power. Please see the related question (link below). The Nazis loathed the Jews and wanted to exclude them from society.
Ghettos
because they did not see a role for these people in their society