tactic since ancient times, but by the time of WW2, the allies and germans had stopped using it, the only ones who still practiced it were the Russians and maybe individual Japanese units.

25%+ is Soviet deaths in the hands of Soviets is probably low estimate. Authoritative numbers was Stalin had at least 600,000 "shtrafbat" (prisoner soldiers), most did not survive. Many others were forced into battle, with "head choppers" stationed behind them in the rear.

Don't forget when Hitler invaded the USSR, most Ukrainian nationalists and many other non-Russians revolted and joined the Germans, those soviet citizen "traiters" were all killed by Stalin during and definitely after the war.

This history was white washed after the war, especially by liberal historians in the west, you can read about it from mainly Russian and east european sources nowaday.