I'm not even sure if that narrative (the killing of surrendering soldiers) conflicts with how Ukraine is portraying itself at this point. The gloves have come off indeed:

From The Kyiv Independent:

⚡️Correction: Ukrainian special forces will no longer capture Russian artillerymen.
The command of Ukraine’s Special Operations Forces has warned that it will not spare Russian artillerymen in response to their “brutal shelling” of civilians and cities.

(thread)

How you approach this statement is entirely up to you; and to be clear, I have not seen any evidence to suggest that Ukraine is engaging in war crimes. However, with soldier's comment and the context provided by Ukraine's SFO, it does raise some questions.

Putin, on the other hand, is verifiably guilty of being a war criminal. One day, sooner or later, he will answer for these abhorrent crimes in a court of law.