The USSR was fighting for its existence in a war of extermination. Sacrificing lives was not really a choice but rather one of the rare advantages they could press.

Smaller Eastern European countries had even more appalling losses, not because of their disregard for human life, but because of their enemies' appetite for murder.

Stalingrad was a worthwhile strategic objective, if I'm not mistaken. Keeping the Wehrmacht engaged also gave the Soviets an opportunity to stage the massive pincer that broke the Wehrmacht's back and damn near knocked its allies out of the war

Dictators tend to be a lot more willing to sacrifice masses of their people to preserve their rule than the subjects are to take that particular path, even if surrendering to the Nazis did sometimes lead to an equally-bad outcome as fighting. But Stalin had just recently purged all potential rivals and opposition so there was no one to stop him internally.