about twenty-one, obviously some were killed on 'Kristalnacht' in 1938, but presumably you meant organised murder.
While the National Socialist Party was a German political party headed by Adolph Hitler there were plenty of Polish, Austrian, Dutch, and even American Nazis during WWII. Not all Germans were Nazis and not all Nazis were German.
The Nazis did not plan to murder all prisoners in camps built before the Holocaust
Blacks and other races were discriminated. Hitler and the Nazis began to really not like Jewish people and began throwing them in Gas chambers or killing them for no reason.
1934
The Nazis did not plan to murder all prisoners in camps built before the Holocaust
According to the German Wikipedia, in November 1944 the SS began dismantling the crematoria in the hope of reassembling them in Mauthausen. Shortly before the arrival of the Soviet Army in Auschwitz (27 January 1945) the SS blew up the gas chambers and the remaining crematoria - and then fled.
The empire of Japan sided with the Axis powers - mainly Germany and Italy.
Systematic, large-scale killings began in 1941. By that stage the Nazis simply took it for granted that the Jews were enemies of Germany, Communists, very dangerous and evil. They did not 'make excuses' or go in for explanations - after all, the genocide was top secret'. They simply did it. Please see the related question.
The genocide began in June 1941 as the mobile killing units went into action behind German lines in the then Soviet Union.
In the late 1930's the Nazis killed thousands of handicapped Germans by lethal injection and poisonous gas. After the German invasion of the Soviet Union in June 1941, mobile killing units following in the wake of the German Army began shooting massive numbers of Jews and Roma (Gypsies) in open fields and ravines on the outskirts of conquered cities and towns. Eventually the Nazis created a more secluded and organized method of killing enormous numbers of civilians -- six extermination centers were established in occupied Poland where large-scale murder by gas and body disposal by cremation were systematically conducted. Victims were deported to these centers from Western Europe and from the ghettos in Eastern Europe which the Nazis had established. In addition, millions died in the ghettos and concentration camps as a result of forced labor, starvation, exposure, brutality, disease and execution.
Routine major deportations began in October 1941 with the deportation of the Berlin Jews. (At the time, the Nazis didn't have extermination camps, so many of the German Jews were dumped in the already overcrowded ghettos in Warsaw and Lodz, but most were sent to Riga, Latvia, where they were shot).In the early stages of the Holocaust the Nazis sent the killers - the mobile killing units - to the victims, but later they transported the victims to the extermination camps, as they found this simpler, less messy and more 'efficient'.
Schleicher, was the last German Chancellor before Hitler,and was in power only 57 days,