The answer to this fundamental question appears to be obvious and straightforward. Nazis and their allies (the Axis Powers) grossly violated one of God’s commandments – ‘You Shall Not Kill’ by murdering millions of Jews and individuals of other nationalities.
National-socialist ideology was fundamentally incompatible with Christianity and the Nazis persecuted the Christian Church (especially in Poland). Consequently, they obviously served the Devil.
However, this is only one side of the matter. There is another side. On the Easter front, Germany and its European allies – Italy, Romania, Slovakia, Bulgaria, Hungary, Croatia and the Spanish ‘Blue Division’ (which included 1,000 Portuguese volunteers) – fought to save the Christian Church and thus the whole Christian civilization from being destroyed (annihilated, actually) by the Bolshevist Soviet Union. Which obviously meant serving God, not Devil.
The Bolshevist ideology that Joseph Stalin used to conquer the world, destroy the global human civilization and replace it with an alternative civilization – global Union of Soviet Socialist Republics – was even more incompatible with Christianity than national-socialism.
Adolf Hitler and Nazis believed in the omnipresent, omniscient and omnipotent God that shared some features with the Christian God. Bolsheviks were atheists and thus denied the very existent of God – any God.
While the Nazis did persecute the Church, they signed concordats with both Catholic and protestant Churches and used violence only when they believed that the other side violated the terms of these agreements by getting involved in politics.
The requirement to stay out of politics did place serious restrictions on what the Christian clergy can say in public, but the Nazis never intended to destroy the Church (Catholic or Protestant).
The Bolsheviks completely destroyed Catholic and Protestant Churches in the Soviet Union and were well on their way to do the same with the Russian Orthodox Church. Only the German invasion prevented them from finishing the job.
The German authorities on the occupied territories actually helped to restore the infrastructure of the Christian Church (Orthodox, Catholic and Protestant) on a scale that seriously worried Bolsheviks who feared that they might lose the propaganda war – and thus the whole war.
Consequently, the only correct answer to this fundamental question is this: actions of the Nazis (like those of many other people) served both God and Devil. But not equally as Nazi Germany never constituted an existential threat to the Christian Church and the Christian civilization.
Nazi Germany and other Axis Powers (obviously, minus Japan) prevented the Soviet Union from destroying the Christian Church and the Christian civilization thus performing a vitally important service for God. Consequently, they served God far more than “the other side”.
It is very important to keep in mind that Germany and all its European allies (with the exception of Finland) fought under the Christian insignia. Germany used the Balkenkreuz (a derivative of the cross developed and worn by the Teutonic knights worn in numerous wars against European pagans).
Subsequently, this cross was transformed into a so-called Eisernes Kreuz – a Prussian emblem that symbolized traditional Prussian values – first and foremost, the Christian religion.
The military insignia of two largest Allied powers – the USA and the Soviet Union – were decisively pagan (white and red pentagram, respectively). And all other Allied Nations used neutral insignia – neither Christian nor pagan. Only Free France (who fought to liberate their country from German occupation) fought under Christian insignia – the Cross of Lorraine.
Finnish Armed Forces and German Luftwaffe used swastika (the latter also used the Balkenkreuz) – the symbol that was extensively used in Christianity until the start of World War II.