Russia lost 50 million killed (military and civilian) during WWII.
The fighting there was brutal. The Nazi SS massacred millions of Russian civilians because they were Jewish or for other reasons, either by lining them up in front of slit trenches that the victims were forced to dig first and machine-gunning them, or by shoving them in the backs of vans and asphyxiating them with an exhaust system that fed into the back of the van.
Stalin was a vicious dictator, but without Russia's help, the Western Allied nations may not have been able to defeat Nazi Germany.