The Holocaust/Perpetrators and collaborators
The killings were authorized by Adolf Hitler and Heinrich Himmler, the main men responsible for the Holocaust and the two most powerful Nazi leaders. A mass of evidence suggests that sometime in the fall of 1941, Himmler and Hitler agreed in principle on the complete mass extermination of the Jews of Europe by gassing, with Hitler explicitly ordering the "annihilation of the Jews" in a speech on December 12, 1941. Prior to this, Himmler had methodically planned out the Holocaust and then told Hitler of the "Final Solution" plan, which Hitler agreed to. Having gotten approval to put his plan into motion, Himmler quickly began a programme of systematic state-sponsored murder of Jews, Gypsies, Soviet prisoners of war, Polish and Soviet civilians, homosexuals, people with disabilities, Jehovah's Witnesses and other political and religious opponents, which occurred regardless of whether they were of German or non-German ethnic origin. Himmler also ordered the Einsatzgruppen ("task forces") to be formed. These were were death squads which went by foot murdering Jews, Poles, Soviets, and just about anyone resisting Nazi rule. Anyone who escaped concentration or extermination camps were hunted down by these death squads.
As to who knew of the mass murders, the concentration and extermination camps, the death squads, and medical experiments that were going on is debatable. Certainly, Adolf Hitler and Heinrich Himmler knew about everything that was happening. Hermann Göring is another high ranking Nazi official who knew, but didn't always approve of Himmler's ruthlessness. While Himmler was intent on murdering all "inferior sub-humans", Göring had always maintained that using the people for slave labor was far more beneficial to the Third Reich. Other Nazi's that knew of the killings were those directly subservient to Heinrich Himmler such as Ernst Kaltenbrunner, Reinhard Heydrich, Adolf Eichmann, Odilo Globocnik, and many SS doctors, guards and officers that worked directly in the camps like Rudolf Höss, Josef Mengele, and many others. The German people as a whole may not have known exactly the degree of mass murder that was going on, but most Germans were aware that non-Germans, especially Jews, were being taken away and persecuted. Most Germans turned a blind eye, and yet many others approved of the persecution of the Jews.