I don’t condone killing surrendering soldiers.
But the Ukes lived through the horror of the Holodomor,
They have memories of the Russians we don’t have.
and act accordingly.
You want to get a flavor of it in America style, read the book Andersonville, it is fiction but lots of it based on real facts, darn close to the reality of it all when you are pissed off at the enemy.
I’d also point out that the soldier (mercenary?) who shot the Russian initially (in this account) “hobbled over” to do it.
That’s no excuse for shooting a prisoner, but, it may partially explain the shooter’s state of mind / lack of clear thinking. (Prisoners are good for intel, prisoner exchanges, etc.)