Wouldn't that mean we would have to a less "pragmatic" (a pragmatic as a paranoid power hungry lunatic can be) Stalin who would be willing to go invade Japanese Manchuria and Korea in the first place. If we remove Hitler from the equation with an early death, we could have something close to Godwin's law with Stalin.
According to Gaddis's
We Know Now, "socialism in one country" is not necessarily a peaceful creed that rejects expansionism in principle.
(And just because the book states that the USSR was expansionist doesn't mean it's reds-under-the-bed. Among other things, it explicitly states that Castro sent the first wave of troops to Angola on his own, not as part of some worldwide Soviet plot, and the Soviets only got involved somewhat later to avoid looking "less revolutionary than thou.")
Furthermore, the Japanese humiliated Russia not all that long before historically speaking. And the Japanese won't exactly have lots of friends, especially if the Anglo-Japanese alliance breaks per OTL. Europe is not going to dogpile the USSR over attacking the Japanese in the same way it would if the USSR attacked Poland or Germany. And going for warm-water ports like Korea would be rehashing traditional Czarist imperialism--it's not something radically new.
Heck, if Stalin were really clever, he'd wait until after the Rape of Nanking. Then he can claim it's a humanitarian gesture and with the KMT having lost its capital, they're not in much of a position to reject his "help."