It was the only reasonable thing to do. Everywhere the jew-communists took power they initiated a Red Terror, during which they butchered anyone (and the families of anyone) capable of opposing them. Germany was lucky enough to have the Friekorps, Hungary was lucky enough to have Horthy, but Russia was already on its last legs when the jews launched their revolution. Pdfrelated has a great chapter on the Red Terror I recommend you read to get an idea of just what the NSDAP was trying to save the world from.

Anyways, the Bolsheviks were happy to admit their goals and any responsible statesman knew they were a threat. Germany was just in the best position to end that threat. Germany and its allies were already bordering the USSR; as the Allied Powers had already been pushed off the continent, more soldiers would be free to fight in the Eastern campaign. Ukraine was considered the 'breadbasket of Europe' and the Caucasus was rich in much needed oil. Germany's colonies had been stolen at Versailles, and while the English and French controlled vast amounts of territory to export their people and goods, overpopulated Germany had no colonies. These colonies were clearly never going to be returned, but the Germans still needed living space. Hitler was of the opinion that it would be better to establish colonies in close proximity to Germany because those would be easier to exploit and integration into the Reich would eventually be possible. Russia, where the just-massacred upper/middle classes had been largely comprised of ethnic Germans, seems the logical target. Take all of this into consideration as well as the fact that Stalin was massing armaments on the border and could easily conquer Romania's oil fields which Germany relied so heavily on, and it should be obvious why they attacked the Soviet Union when they did.

Generalplan Ost is nothing more than the speculation of Brits/Americans/Jews trying to justify their genocide of Germany. In reality, as there is no documentation, we cannot know what was planned. However it seems more logical to me that Germany would slowly colonize Western Russia, sterilize/deport the worst slavs, germanize the best, and grant the rest second class citizenship. Integration probably would have been completed within a generation or two.  

Lot to unpack here.

Hitler as he discussed in detail in Mein Kampf wanted to create a German superstate capable of rivaling the United States in terms of its economy and population. To do this, he proposed something we call "lebensraum".

Germany was too small population wise to ever be a global power. So, the idea was that Germany would conquer Eastern Europe (which was filled with Slavic peoples that Hitler considered to be "subhuman"), kill/deport most of the adult native population, have ethnic Germans settle the area, and eventually through breeding "aryans" enough, have an ethnostate controlling all of central and eastern europe.

So a conflict where Germany attacks Russia is inevitable. But why attack the Soviet Union when they did? The Molotov-Ribbentrop Pact heavily favored the Soviets in the long run, and both sides knew this. The most lopsided part was how trade was conducted. Germany NEEDED oil, and they pretty much could only get it from the Soviets. That meant the Soviets could change the cost or cut the supply altogether whenever they wanted which is pretty dangerous for the Germans. Both sides knew that the agreement would only last a few years. The Germans, after realizing that the British weren't going to surrender, decided to attack the Soviets in late spring 1941. This was pretty much the best time they could attack. Before then, the Soviets wouldn't have begun the massive rearmnament and reorganization of the Red Army. And after then, the Red Army would be completely reformed and the Germans wouldn't stand a chance.

Got off topic but does that answer your question?  

To kill commie jews, liberate slavs, and secure the lands of germans in ussr. He also did it with a small hope Churchill would see the Reich was not interested in invading GB or dismantling her majesty's empire, but in finishing what the West failed to do in the Russian Civil War: killing bolshevism.