I don't think that the East wanted to separate from Ukraine (the only part of Ukraine for which the question of the separation/reunification with Russia was always relevant was Crimea), but they were always pro-Russian — while the West was always more, well, Western-oriented with the main roots of Ukrainian nationalism (not in Putin's understanding) coming from there, with Lviv as its central hub of sorts. Kyiv was the most balanced city where both of those views were widely represented.
For example, this is the split from the 2004 election, the first one where the Ukrainians were essentially choosing between the West (Yushchenko was trying to move towards the EU and away from Russia) and Russia (Yanukovych was always extremely close with Kremlin):
This is from 2010, Tymoshenko was a prime minister under Yushchenko, Yanukovych is still closely connected to Moscow — as you can see, the results are almost the same (although Yanukovych wins this time):
Obviously I expect that all of those bright red regions had turned blue/deep blue by now.