Why did Hitler persecute others?

'Under the cover of the WW2, the Nazis tried to kill every Jewish man, woman and child in Europe.
For the first time in History, science and technology were not used to improve peoples’ lives but for the mass murder of a whole people. Six million Jews, including 1,500,000 children were murdered: this is called the Holocaust. The Nazis also enslaved and murdered millions of other people because of racism, intolerance and prejudice. Roma and Sinti people (sometimes called Gypsies), people with disabilities, Poles, Soviet prisoners of war, Jehovah’s Witnesses, homosexuals and others were killed in vast numbers.
None of this happened very long ago: some of the people involved are still alive today. Nor did it happen very far from where you live. These events occurred in the ‘civilised’ countries of modern Europe. So it was not only the lives of people at the time that were torn apart by the Holocaust, but also our ideas about how human beings treat each other. Because of this the Holocaust is important not only as an event in history but also for how we live our lives today.’
 
Extract from ‘Torn Apart’ by Imperial War Museum