I keep hearing about the false flag operations in Bucha and it made me wonder why the Russian Army is pulling back from Kiev at all?

Are they regrouping? Or was the only objective to destroy the bioweapons labs? I just don't understand why they would leave without fully removing Zelenskyy and finishing the denazification. All I can find from reliable sources just point out how the Ukrainians creating fake masacres, but nothing about why the Russians are letting Ukraine take some of the ground back.