The answer to this question is also not as simple or straightforward as it might seem at first glance. On the one hand, the Soviet Union fought to liberate its territories and East European nations from the brutal German occupation and thus put an end to mass murders and other crimes committed by the Nazis. Which was, obviously, a vital service to God.
It should be noted, however, that the Soviet Union provoked these very mass murders and other crimes by instigating large-scale guerilla activities on the occupied territories of the Soviet Union, Poland, etc.
And, of course, the Soviet Union (and personally Joseph Stalin) fought to conquer Europe and the whole world, annihilate the Christian Church, destroy the global Christian civilization and replace it with the alternative civilization (global Union of Soviet Socialist Republics). Civilization based on the atheist Bolshevist ideology totally incompatible with Christianity.
Consequently, the only correct answer to this fundamental question is this: actions of the Soviet Union (like those of many other people) served both God and Devil. But as the USSR constituted an existential threat to the Christian Church and the Christian civilization, its actions served Devil far more than they served God.