So in no way am I suggesting this is the case, but after my previous post saying I couldn't think of any state actors that would gain from this, a possible cui bono occurred to me which is that if Western powers wanted to disengage from the war somewhat then a false flag directed against Ukraine would give them the catalyst to distance themselves slightly from Kiev. It wouldn't even need to be wholly a false flag as Ukraine has plenty of elements that are motivated enough to do this, it would just be done directly by Western handlers without the sanction of Kiev. Then in the next few days or week we would see statements from the West about how they don't condone such behaviour and it is time to reassess and have discussions with Russia directly, perhaps rather than Kiev.
There's no evidence, I'm just correcting my earlier post that I could see no gain for any of the players in doing this. Factors relevant to this hypothesis are: It is a certainty that the West knows now that they're not getting back Crimea anytime soon and unfeasible in the extreme that they'll reclaim Donetsk or Luhansk. I think with the talk of a French led occupation of Western Ukraine that they're now looking for a way to stall things as quickly as possible. And probably cut Zelensky out of the loop as there's a real motivation for him to keep on fighting - he might literally be killed by Ukie ultranationalists for backing down to Russia or Ukie military leaders might politically supplant him. Biden has staked a lot politically on backing Kiev every step of the way and it is an election year. He needs an out. Ukraine perpetrating a massacre against civilians gives him one. Maybe.
If this isn't Western backed then it will be memory holed / denied even if Ukrainian elements were involved. If it is Western backed then you'll see Western condemnation and talk about how the West needs to directly involve in guarding Western Ukraine and de facto, they'll supplant the Kiev government. They'll need to do this quickly before Russia eats up any more land.
Again, not arguing for this and interested if there's any obvious flaws in it, just theory-crafting. Thoughts with those who've lost people in this and to those injured.
EDIT: Well, ISIS claim has apparently come in whilst I was writing all that.