Part 1

Several things need to be taken into account for an answer here.

First, not all "undesirables" were to be systematically killed. Political enemies, so-called "asocials", criminals, homosexuals, Jehovah's Witnesses were first and foremost to be imprisoned and secondly to be exploited economically, especially after the two waves of Concentration Camp economization in 1938 and 1942.

Many of them dies and suffered horribly but for the Nazis, these people were not on the list to be systematically murdered wholesale. Amidst the general lack of work force in the Nazi war economy, the labor of Concentration Camp prisoners became a rather valuable resource, in the sense that the SS started their own companies (mostly in 1938) and later on started to rent or lease Concentration Camp labor to big companies in the Third Reich. In the Auschwitz camp complex, Auschwitz III Monowitz was a camp that basically existed only to provide the IG Farben -- the biggest chemical company in the world at that point -- with cheap prison labor. Of course, the working and living conditions were horrifying and thousands upon thousands died of starvation, maltreatment, disease, and of being executed. But the initial plan was not to have them killed wholesale.

Four particular groups were singled out by the Nazis to be killed wholesale: Physically and mentally disabled within the borders of the German Reich, who were in corresponding institutions, Jews, so-called gypsies (Roma, Sinti and others who were caught under the umbrella term), and Soviet POWs in 1941.

With Soviet POWs, the initial plan was to let them starve in order to save food to feed the German Wehrmacht and the German populous. In 1941 alone 1.5 million of a total of 3 million Soviet POWs stared to death and all in all about 3.3 million Soviets in German captivity died throughout the war out of a total of 6 million. Starvation was chose here because it was the "easiest" route to take and it didn't require anything more than putting them in camps and watch them die. This policy however changed when it became apparent that the Germans weren't going to win the war in the Soviet Union in 1941. Due to the labor shortage the war caused for the German economy, it was decided that the should work for the Germans rather than starve wholesale. And while, in later years, there still occurred mass deaths of Soviet POWs in Germany, this was more the result of a policy of intentional neglect of their basic human needs, i.e. indirect mass murder, rather than a campaign of seeking them out to kill them.

For the mentally and physically handicapped, the rationalization of the Nazis to kill these groups was ideological -- to get them out of the gene pool -- as well as economical -- to not have to feed and care for them anymore. But because this group was mainly German citizens with German relatives and the Nazis were highly conscious of popular support during the war, the initial program set up to kill these "unproductive eaters", the so-called T4 program, was done in secrecy. Shooting them was a highly public act, the German population would have noticed. This program is where the method of gassing as a way to mass kill people originated.

Two things are important here: First of all, when Hitler, Philipp Bouhler of the Reich chancellery, and Karl Brandt, Hitler's doctor, decided to bring the T4 program underway in 1939, they decided that only those within appropriate institutions (hospitals, homes, and so on) were to be killed because arresting the handicapped in their homes dragging them away from their families directly ran contrary to their need for secrecy. Secondly, because of said need for secrecy, shooting was out of the question. Therefore, they asked the Criminal Technical Institute of the Security Police (Kriminaltechnisches Institut der Sicherheitspolizei, KTI) what the fastest, most-effective, and time-saving method of mass killing was, they could come up with. The KTI ran several experiments, first on animals, but then in October 1939 they also experimented for the first time on mentally ill people in Posen, where about 400 people were gassed with Carbon-monoxide from gas bottles. Witnessed by August Becker, later to be the inspector for the Gas Vans in the Soviet Union, the KTI was apparently satisfied with that method, especially with how it was possible to kill a lot of people in a short amount time with a resources that was easily available because every engine could produce it. Thus they recommended this method to the planners of T4.

During 1940 the T4 program also conducted their own trials of which method of killing was the most effective. While gas chambers and crematoria were established in six main T4 facilities -- Brandenburg, Grafeneck, Hadamar, Bernburg, Hartheim, Sonnenstein --, the worries about keeping the program secret were well founded on the side of the Nazis because when the Catholic Church started publicly preaching about the injustice of the program, the Nazi government saw itself forced to publicly stop the program. While the killing of the mentally and physically handicapped became a decentralized program with individual doctors killing their own patients instead of sending them to one of the central facilities, the gas chambers in the T4 facilities were still used for program 14f13. 14f13 was the deliberate killing of Concentration Camp inmates who had become to weak or sick to work. The name comes from the Concentration Camps' forms on causes of death. 14f13 stood for death by diseases and during some phases it can be read by historians today as proof that the people were gassed.

For Jews and the so-called gypsies, the development of policy was different. The first mass executions of Jews took place in Poland in 1939 when the Nazi Einsatzgruppen killed Polish intellectuals and Jewish political enemies in the thousands as part of their campaign to deprive the Polish people from any sort of intellectual leadership that could lead the Polish resistance or a resurgence of the Polish nation.