I would say these people aren't POW's because there is no war. It's an illegal invasion. Ukraine isn't at war with Russia.
The Geneva Convention is drafted so as not to allow these kinds of shenanigans: they are POWs. Even if they weren't, that would make them illegal immigrants subject to domestic law not summary execution.
In the big scheme of what is going on in Ukraine, I find it hard to get particularly worked up over it, but it's still not good. It shouldn't happen, and I hope it's properly investigated and dealt with by the Ukrainians (who, to be fair, have condemned these actions).