Someone on that Twitter thread (Count, perhaps?) posted this:
"I'm sceptical, it is a very convenient narrative for the Kiev regime, just as they are facing a total loss against Russia. It may or may not be true, but sounds to me like a desperate attempt to rally wavering allies!"
Personally I'm inclined to believe the worst about the Russians, especially the Wagner Group mercenaries who are apparently behind this. I also, against my better judgement, found the video in question and wished I hadn't, having experienced this many years ago when Al-Kaida posted a video where they similarly beheaded a western journalist. I watched that video and instantly regretted it and today was no different. If it's staged it's one hell of a realistic set-up. I fear that it's real.