Nazis believed in the "stab in the back" theory for the WW1 defeat of Germany. This was a conspiracy theory according to which WW1 had never been lost on the ground by the German army but was lost because jewish and bolchevik politicians betrayed the German army and negociated a peace in its back. In their views, the jews were the most dangerous race of subhumans because they corrupted everything from the inside. It is therefore no wonder that the nazis thought they could not conquer anything if they had the jews in their back free to plot and undermine everything they were trying to do.

Concerning the method of killing. The nazis made a lot of research to try and find the most efficient way to kill and dispose of the bodies. They did try shooting them directly but the ammo cost was high (they were trying to wage war, wasting millions of bullets killing jews was not good when some units on the front needed them) and they also had many problems with soldiers refusing to shoot unarmed civilians. You also have to find a way to dispose of the bodies somehow, shooting a few dozens of civilians in the countryside would probably allow you to leave them there but as soon as you start going industrial you have to find a way or you are going to find yourselves with literal mountains of bodies.

The camps were supposed to be the answer. They first used gas engine exhaust fumes but the gas was hard to come by and the tanks needed it, also it was not efficient and a lot of jews would wake up afterwards. After a few more tryouts they ended up using Zyklon B, a super common pest killing gas Germany was already producing tons of.

So basically they needed to get the jews out of the equation quick, they could not leave them in the general population for fear of them plotting in the shadows, spying and basically anything jews did according to the nazis.

Source: The Destruction of the European Jews - Raul Hilberg

Germany and the Second World War: Organization and Mobilization in the German Sphere of Power, Wartime Administration, Economy, and Manpower Resources - Rolf-Dieter Muller,Hans Umbreit, Derry Cook-Radmore, Bernhard R. Kroener

I won't write about the ways in which the Holocaust came into being, but I'll try to explain the Nazi worldview in which the destruction of the European Jewry was inextricable from the conflict in Europe.

For Hitler and many other Nazis, the war and their campaign against the Jews were inseparable. They believed that the Western Allies were under the control of some kind of Jewish-financial cabal, while the Soviet Union was seen as the headquarters of Judeo-Bolshevism. This is especially the case in the Eastern Front, where the Nazis didn't think that they were just waging a war against the Soviet state (and where the Holocaust was actually initiated). Instead, Hitler and the Nazis believed that the conflict against the Soviet Union was a great race war between the Aryan, Germanic peoples of Europe against the Judeo-Bolshevik rulers of numerous Slavic and 'Asiatic' races. For Hitler, communism and Jews were inseparable, which has obvious implications in his campaign to eradicate Bolshevism in the Soviet Union.

As soon as German soldiers crossed the Soviet border in 1941, the Nazi campaign to wipe out Judeo-Bolshevism began, however it would be some time until the outright mass-execution of Jews would go underway (Einsatzgruppen did begin the mass murder of Jews early on, but it was done under trumped up charges of criminal behaviour and wasn't rationalized because the victims were Jews and needed to be killed, though this is obviously the intent behind the massacres). Hitler's Commissar Order required that Wehrmacht troops summarily execute Soviet political commissars without trial immediately upon their capture; despite breaking international military law, the Nazis believed that this measure was a necessary first step in their campaign to extirpate Jewish-Communism in Europe.

Because the Nazis understood their war in the east as a campaign against Jewish communism, all Jews in occupied territories were immediately suspected of being potential partisans or resisters. von Reichnau's Severity Order for soldiers in the 6th Army from October of 1941 established this connection as an operational fact in the German occupation of Soviet territory:

Regarding the conduct of troops towards the Bolshevistic system, vague ideas are still prevalent in many cases. The most essential aim of war against the Jewish-bolshevistic system is a complete destruction of their means of power and the elimination of Asiatic influence from the European culture. In this connection the troops are facing tasks which exceed the one-sided routine of soldiering. The soldier in the Eastern territories is not merely a fighter according to the rules of the art of war but also a bearer of ruthless national ideology and the avenger of bestialities which have been inflicted upon German and racially related nations.

Therefore the soldier must have full understanding for the necessity of a severe but just revenge on subhuman Jewry. The Army has to aim at another purpose, i.e. the annihilation of revolts in hinterland, which, as experience proves, have always been caused by Jews.

The combating of the enemy behind the front line is still not being taken seriously enough. Treacherous, cruel partisans and degenerate women are still being made prisoners-of-war and guerilla fighters dressed partly in uniform or plain clothes and vagabonds are still being treated as proper soldiers, and sent to prisoner-of-war camps... If isolated partisans are found using firearms in the rear of the army drastic measures are to be taken. These measures will be extended to that part of the male population who were in a position to hinder or report the attacks...

The fear of German counter-measures must be stronger than threats of the wandering bolshevistic remnants. Regardless of all future political considerations the soldier has to fulfill two tasks:

1.) Complete annihilation of the false Bolshevist doctrine of the Soviet State and its armed forces.

2.) The pitiless extermination of foreign treachery and cruelty and thus the protection of the lives of military personnel in Russia.

This is the only way to fulfill our historic task to liberate the German people once and for all from the Asiatic-Jewish danger.

Though later rescinded after his death, Reichenau's order set an important precedent in the German occupation of the east: Jews, even unarmed civilians, were to be automatically regarded as elements of resistance in the German race war. The order was merely the logical conclusion of Nazi racial ideology.

So even before the birth of the 'Final Solution' the logic behind the systematic extermination of the European Jewry was in place: the war was an immense racial war between Germany and various Jewish cabals; representatives of Jewish-communist ideology were to be summarily executed upon capture as a necessary part of the struggle and most importantly; all Jews were to be regarded as militant representatives of the Judeo-Bolshevik threat. For the Nazis, the utter destruction of all Jews in Europe was an inevitability as soon as their declaration of war on the Soviet Union, and the USA's declaration of war on Nazi Germany.

Edit: to be clear: the concept of Judeo-Bolshevism was a conspiracy theory with no basis in reality. I'm not advocating that there was actually a Jewish-communist conspiracy that needed to be destroyed; I'm merely conveying what the Nazis believed.

Okay I'm seeing some bad or misleading information here, so i want to clarify a few things. Before anything though, its important to note that the the death camps and more broadly the final solution itself were not explicitly planned by the nazi regime from the get go. While, they certainly wanted to rid Germany and their newly acquired territories in the east of Jews, extermination did not become official policy probably until sometime in late 1940 or early 1941. While it is certain the hitler knew about the holocaust and approved it, the "smoking gun" or direct order has not been found and probably does not exist.

Secondly, I saw a user mention "uniqueness of the industrialized slaughter" and "innate bloodlust." I personally do not like the uniqueness argument but I can let this pass, as their are good arguments on both sides. However, of the roughly 6 million jews murdered, the vast majority (4.5-4.8) million were shot in mass graves.

Finally, to call the perpetrators of the mass slaughter "blood crazed" is to misrepresent the complexity of the situation. We know from social science that it is extremely difficult for the average person to kill someone they view as innocent or helpless person. While the slaughtering of men could be rationalized as the killing of enemy nationals or partisans, but even the self-selected and fanatical SS men could not cope with the horrific reality of murdering women and children. Alcoholism and alcohol abuse was rife among the death squads, many broke down and were "reassigned."

Moreover, those SS that were overeager or enjoyed the killing were also removed. Driven by himmler's beleif that the SS were the epitome of the aryan race and the german people, the extermination of European Jewry was a necessary evil and burden and sacred duty. Within his bizzare nazi framework, killing innocent men, women and children was permissible as long as it was not enjoyed. For all his talk of duty and bravado, Himmler would be the one to order the exploration of alternative methods of mass execution after witnessing his first mass murder in Poland. Shooting was not as clean as he had previously thought. Many remained alive after the initial round(s) of machine gun fire and had to be dealt with personally, blood and body parts littered the surrounding areas. He realized he needed to find a more "humane" way to kill. Not out of any compassion for the victims but to save his men from the mental anguish.

About this time. the Nazi medical establishment had begun to experiment with gas for their forced euthanasia program. Carbon monoxide seemed be too inefficient, but their seemed to be promise in a new gas called Zyklon B...

Sauce

Bloodlands: Europe between Hitler and Stalin - Timothy Snyder

KL: A History of Nazi Concentration Camps - Nikolaus Wachsmann

Ordinary Men - Christopher Browning

you are feed information like a child from the news etc. obviously you are brainwashed you believe everything usa tells you so in other words you are there slaves because you don't choose your beliefs they choose them for you. but nothing on news is actually proven you think you know what proof is but truly it's not on the news or internet. funny how people such as admins believe what there feed but don't know the truth for themselves they use assumption, judgment & guessing & that's what proof is to you. you dumbies don't know the truth but be the slaves you always have your destiny is to be deceived over & over by usa.