Yanukovych was corrupt and an authoritarian. Putin doesn't like pro western governments.
Yanukovych's political turn toward Russia and away from the European Union in late 2013 triggered massive pro-democracy protests.
Yanukovych was recognized as the legitimate democratically elected leader by both east and west. Every post-soviet republic was rife with corruption and authoritarianism, and that only got
worse under the new US/EU aligned regime when the oligarchs simply switched to paying off US/EU leaders like Joe Biden and the state started banning the opposition, murdering dissidents and journalists and stopped holding elections entirely.
Yanukovych had an obvious national Ukrainian interest, rather than personal or corrupt, in appeasing Russia and keeping ties with Russia. Because he understood the lengths Russia would go to defend its own back yard.
If the opposition to Yanukovych wanted democratic change, they only had to wait a matter of months and vote him out in the next cycle.
But that didn't happen.
Yanukovych won because the RU-leaning sentiment was the majority, the ethnic russian oblasts more populous. Those who overthrew him were actual neo-Nazis and passed out copies of Mein Kampf in Maidan Square. They were an insurrectionist minority who set off a civil war they could never win on their own, purposefully appealing to foreign masters to turn it into an east vs west proxy conflict, so much for Ukrainian sovereignty. So much for "pro-democracy", they stopped holding elections.
The Ukrainian revolutionaries only conquered the west, and since then they've been fighting a war while laying a pretend claim to an east they've never held for a single day since 2014.
If they dropped their absolutist demands and agreed to the status quo borders, they could maybe even return to being a legitimate democracy in time, where the people of west Ukraine have self-determination.
If tomorrow the separatist regions all inexplicably surrendered and submitted to rule by the west, would they be a "democracy"? The last vote they held as a unified country, the east won.
There's no calculus where this adds up to anything legitimate for west ukraine claiming the east anymore.