So, set aside legal or ethical concerns for a moment.
Let's say that when you capture soldiers, that you adopt a policy of placing them in a situation that'll kill them off. Because, sure, you're upset with 'em for participating in the war.
What that does is ensure that soldiers who might have surrendered are going to fight to the death. If a soldier is dead either way, then there's no meaningful surrender offer.
Ukraine -- just as any other country at war -- doesn't want to have to fight every Russian soldier to the death, as that results in a higher cost in Ukrainian lives. Ukraine would considerably rather that a given Russian soldier who is willing to surrender actually surrender, not try and discourage him from doing so.
