1. to get rid of Jews, Sinti, Roma and all the other low level ethnic groups in the eyes of the Nazis
2. expand living space (Lebensraum) to the east -> it was clear for the Nazis, that Russia was the place to be
3. Lead the German master-race to a major global player
3. maybe the west, because he was "surprised" that France and England declared war after the Nazis invaded Poland
So defeating Russia was not an obsession, but a necessary factor to expand to the east.
SF