>is western Ukraine, full of ethnic Russians, which was not part of Ukraine until relatively recently, should be considered a proper part of Ukraine; or is the current situation is the result of a bad decision by a single soviet leader, and Russia is not in the wrong to try and change the borders?
Unfortunately the issues are more complicated. From the most unbiased historical perspective that I can reasonably give, some
Russians a long time ago, and have had
historically good ties with the Russians due to both being vassal states to the Golden Horde. Southern and Eastern Ukrainians are either ethnically Russian or are predominantly and culturally Russian until you hit Odesa, Western Ukrainians are Austro-Hungarian, and Northern Ukrainians are sort of a mixed bag but predominantly Austro-Hungarian from the times of the Principality of Galicia. Under the yoke of the golden horde, Ukraine was considered its own state since it is something of the "birthplace" of slavic ethnicities, but for all intents and purposes it was an autonomous satellite region to the Rus princes to the East to keep out foreigners. From about the 1800s to the early 1900s Ukraine was Russian territory, and under the Soviets the Ukrainians more or less maintained the same status quo they had been maintaining since about 900AD as an autonomous region that reported to Russia but was not necessarily subservient to Russia. More or less all slavic groups originate form Ukraine before the coming of the Golden Horde, and have maintained themselves as a mix of Slavic, Iranian, and Turkish ancestry since those times with more or less of one of those three ethnicities mixed in depending on where they were in Ukraine greater. As can be cross-referenced from these maps and the maps I posted in >>21633
the ethnic maps and the geographical maps "kind of" line up but it's not the full story. It's not helped by the fact that Russia is a civic
state and not an ethno
state so while ethnicity might play into the greater picture, it's not the end-all be-all for the Russians.
Reasonably, if you drew a a straight line from Kharkiv to Kremenchuk, everything south of that, on that side of the river, could reasonably be called ethnically Russian lands whether the source of that Russian ethnicity is legitimate (interbreeding) or not (artificial migration). Ukraine lacks clearly defined borders between the ethnically Russian and ethnically Ukrainian parts of the country, so splitting them would be like the India-Pakistan split. You would still have Russians in Odessa and you would still have Ukrainians in Chernihiv, for instance. Unfortunately all of Ukraine's money lies in the ethnically Russian parts of Ukraine
so they are kind of at a stand-still. You have Odesa who does not want to be Russian or Ukrainian but the Ukrainians subjugate them because Black Sea access is necessary for industry. The northern parts of Ukraine have been trying to recover since their industrial heartland to the South-East is in a civil war (and failing because the money/talent/infrastructure just isn't there in Northern Ukraine), and you have North-Eastern Ukraine which is very clearly Ukrainian and does not want to be part of Russia, but which is on the wrong side of the river. I don't mention elections because the 2019 elections were a Russia-tier rigged shitshow that are being used as Ukrainian anti-Russian propaganda, but the previous elections speak for themselves with Southern Ukraine standing firmly and overwhelmingly in opposition to Zelensky. It's more likely to think of the Southern regions as voting "no confidence" rather than voting for the Ukrainian side in 2019.
I think Eastern Ukraine has a right to self-sovereignty and the Russians are more likely to provide that. I think Southern Ukraine does as well. I don't think there is any way to establish a clear border though. Any established border is either overwhelmingly unfair to Ukraine as it cuts off 80% of the country's wealth sources while giving Russia a monopoly over large swaths of the Black Sea, or overwhelmingly unfair to ethnic minorities living in said wealth source regions as the next natural step of the Ukrainian government then would be to start genociding them (legally) by importing ethnic Ukrainians to take over industry and prevent a repeat of any conflict that comes about by ensuring anti-Russian insurgents are embedded firmly in the region.