Here's something straight from Putin's mouth, reported in Yahoo:

"During the talks in Istanbul we managed to achieve quite a significant breakthrough, because our Ukrainian colleagues decided not to make Ukraine's international security guarantees conditional on such issues as internationally recognised borders of Ukraine, essentially bracketing off - with certain caution - [the issue of] Crimea, Sevastopol, and the Russian-recognised Donbas Republics."

According to the President of the aggressor country, after these agreements have been reached, Russia was faced with the situation in Bucha, Kyiv Region [where Russian troops killed hundreds of Ukrainian civilians], which "the Russian army has nothing to do with," as Putin claimed. Putin complained that after the Bucha atrocities the position of Ukrainian negotiators on further settlement has changed dramatically. According to Putin: "They [members of Ukraine's negotiating delegation] have left behind their earlier intention to separate the questions of security guarantees and the issue of the territories of Crimea, Sevastopol, and the Donbas Republics. They just abandoned it. In their proposed agreement on this issue that they submitted to us they simply noted in two articles that these questions have to be resolved in a meeting between the leaders of the two countries. It is clear that if we defer these questions to the heads of state, without at least preliminarily agreeing on them within the proposed agreement, they will never be resolved. Under these circumstances we simply cannot sign [Ukraine's] security guarantees without first resolving the question concerning the territories of Crimea, Sevastopol, and the Donbas Republics."

Virtually no progress has been made since.

Given the totality of evidence on Bucha at least makes the perpetrator unclear (and, as I understand, based on current evidence from the rounds used and the motivations of each party, is more likely to be Ukraine than Russia), if one assumes that it was Ukraine or "rogue" Ukrainian Nazis that did it, then one has to wonder at the motivations. Do the Ukrainian authorities and negotiators genuinely believe that Russia did Bucha? Do they know that it wasn't Russia, but now they've dug themselves into a diplomatic hole, where making peace or surrendering would look bad and cowardly to other people knowing that everybody in the west thinks Russia did an intentional massacre of civilians?