Russian convicts recruited to fight in Ukraine were warned they'd be shot if they deserted.

[Chainfire]

1. A criminal army fighting for a criminal leader of a criminal nation in an illegal war.

It increases morale and discipline to threaten to kill anyone who shies away from the fighting. They learned that trick from Stalin's order number 227.

Personally, as far as I am concerned, the more Russians that the Russian backing forces kill, the fewer the Ukrainians will have to. What I do suspect is that the policy makes the Russian troops look for any good opportunity to surrender. I hope the Ukrainians are smart enough to encourage them to surrender, and then to treat them well; the word gets around.