The vast majority who died because of Stalin's incompetence and cruelty.
One of the great ironies of WW2 is that in 1939, Stalin believed France & Britain were not serious about fighting Germany themselves, and only wanted him as an ally to defeat Germany at the cost of copious amounts of Russian blood. So Stalin, the man who trusted no one, trusted the one man no one should have trusted: Adolf Hitler. He cut his deal with the Non-Aggression Pact. After Hitler dispatched the French, he turned on the USSR. Thus, Stalin wound up doing that which he had tried to avoid; the defeat of Nazi Germany at the cost of copious amounts of Russian blood.
Yes, thanks to the purges and the brutal system of command, the Red Army was club rather than a rapier. But that’s probably what it was designed to be. While they gained tactical and operational competency from 1944 on, the Germans still inflicted immense casualties on the Soviets. And keep in mind, the Germans never had less than 65% of the Wehrmacht on the Eastern Front. Forces we didn’t have to face because of the participation of the USSR.
Bingo. Stalin didn’t care one iota about his people. For Stalin, the millions of dead Soviet soldiers were conveniently expendable. Fewer mouths to feed.
I wouldn’t say vast majority, or even majority. Those Nazis were good at war. Unless you are including Stalin’s prewar incompetence and cruelty, then yes. Purging his officers and terrorizing the entire population but especially certain minorities who ended up abetting the Germans (cruelty), also not seeing the invasion coming even after all the intelligence was in (incompetencin, did add to the death toll considerably.