Is it possible for the Soviet Union to end up in a Civil War again? Let say Stalins purge fails. How does this affect WWII?
Mass German air attack sending literally the entirery of the Lufftwaffe with the sole intent of razing the Kremlin and killing Stalin?
Is such a thing theoretically possible?
If you are allowing for something within OTL WWII then I think various decisions of the Germans could have influenced nationalist sentiments in the Ukraine, Baltics, and the Caucuses that would see large factions (with our without direct German assistance) spring up into something resembling a civil war. Of course, given this takes place along side a German invasion I'm not sure that qualifies as a purely civil war.
If you are strictly asking about some kind of pre WW2 civil war, that's more difficult after Stalin consolidates power. It would be stretching, but perhaps some of the starvation tactics of the Soviets in the Ukraine push some peasant revolts into more widespread revolts with local militias.
This would require the Nazis to not be genocidal maniacs intent on exterminating the peoples who live in those countries, so not likely unfortunately.
There were minor revolts and disorganized banditry during the Great Famine but unfortunately all the peasants are disarmed and starving while the state security apparatus controls all the guns and food.