What war crime? Specifically please. I don’t think there is one involved with their capture. And if killed, executed, or whatever, is this a war crime and, again, if so, how so?
If the men are volunteers for the Ukrainian military then they’re covered by the Geneva Conventions.
If Putin has them executed then he can find himself dancing at the end of a rope.
That’s how this works.
They aren't mercenaries, but UA soldiers who fought in UA uniforms and under UA command.
The only actual mercs fighting in Ukraine are Russia's Wagner group, plus whatever Syrians or Chechen soldiers not integrated into the Russian army.