Contrary to a (very) popular misconception, Heinrich Himmler was the most powerful individual in Nazi Germany (after Adolf Hitler, of course) not only because he controlled political and criminal police and concentration camps and had accumulated enough kompromat to bring down just about anyone in Nazi hierarchy.
But also for an entirely different reason. Contrary to another very popular misconception, the whole Nazi project was essentially about three fundamental objectives: (1) transformation of German people into the nation (“race”) of genuine Übermenschen – “super-humans”; (2) conquest and “pacification” of sufficient Lebensraum (“living space”) in the East to make Germany self-sufficient in foodstuffs and raw materials – and to support the comfortable, affluent and happy life of all Germans; and (3) the so-called “resettlement”. Everything else was a mere means to these three ends.
“Resettlement” was (like many other Nazi terms) a euphemism for an unprecedented campaign of colonization, enslavement, ethnic cleansing and mass murder on the previously unheard-of scale.
In practical terms, it meant (a) enslavement of the native population deemed functionally valuable to the German conquerors; (b) deportation or physical elimination – mass murder – of those deemed insufficiently valuable; and (c) resettlement proper of ethnic Germans on the conquered Eastern territories – in Poland, Ukraine, Belarus, Russia, etc.
The SS was the primary (“lead”) agency responsible for achieving all three key objectives (raison d’être of the Nazi state). Which automatically made it the most important and valuable (and thus the most powerful) organization in the Third Reich – and (according to the omnipresent Führerprinzip) made Heinrich Himmler the most important and valuable (and thus the most powerful) individual in Nazi Germany.
This power was exercised both through his position (and authority) of SS Reichsfuhrer (in particular, via RuSHA – SS Race and Settlement Main Office) and his power as the Reich Commissioner for the Consolidation of German Nationhood (he convinced Adolf Hitler to appoint him to this position on October 7th, 1939 – a mere month after the start of the Second World War).
In the latter position, Himmler was responsible for (1) the return to the Reich of the Volksdeutsche (ethnic Germans with foreign citizenships) and Auslandsdeutsche – German citizens who live abroad; (2) prevention of “harmful influence” of populations alien to the German nation/race; and, last but not the least (3) creation in the conquered Lebensraum of new populated areas settled by Germans, mostly by the returning ones.
Himmler’s value, importance and power were further strengthened by his two other key achievements. First, he positioned the SS as the natural “breeder” of the elite (super-Übermenschen) who will run the Nazi state and the whole Nazi society – the Party, the government, the industry, the academia, etc… and the German Armed Forces. Especially the German Armed Forces.
The second achievement stemmed from the irrefutable fact that Adolf Hitler and other Nazis sincerely (and incorrectly) believed that Germany was fighting a genuinely existential “racial war” with the “Jewish race” (more specifically, with the “Judeo-Bolshevist” monster).
They were right on the second count (the war with Bolshevism was, indeed, an existential one) but dead wrong on the “Judeo” one. Hence, it was much worse than a crime – it was a (genuinely) horrific blunder… but Adolf Hitler and the Nazis sincerely (and incorrectly) believed that to win this existential war, they had to physically exterminate the Jews down to the last man, woman and child.
And again, the SS (more specifically, the RSHA and the SS-WVHA – SS Main Economic and Administrative Office which ran the system of KL and the death camps) was appointed as the lead agency (“general contractor”) for the murderous project that ultimately became known as The Holocaust.