It was Nazi ideology that exterminating what they saw as dangerous and disgusting people would be improving the world. It was very well propagandized into popular thought, and antisemitic views were held by many worldwide at the time. The Nuremberg Trials are very eye opening because you see that most everyone tried was normal, sane, and reasonable, but many had committed atrocities beyond our imagination. The ideology is obviously flawed from our modern standpoint, but it serves as a warning that regular and otherwise moral people can be susceptible to dangerous views.