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.