At this point, I think it's important to focus on the historical events in other countries.
Tbh, what the UPA did was no worse than what the American settlers did in the 18th century - or other purges and genocides in other countries.
The US began with the attempted genocide of the Native American people. They also used very seriously inhumane methods to kill them. Now the Native Americans are 2nd class citizens in the US. I see a parallel in Ukraine.
Throughout history, Ukrainians have been controlled by Polish-Lithuanians, Austrians, Germans, and Russians but they never had the chance to control themselves... until recently. I think Bandera was the breaking point.
So, maybe the Ultra-Nationalists were feeling a lot frustrated?
https://www.youtube.com/watch?v=pmUTBDuUGz8
It's what makes me believe that the Ukrainians are a unique culture. And I also believe that the Ultra-Nationalists are a minority in that culture - like the KKK is a minority in American culture. But do KKK members control the government? Who knows? Memberships in radical organizations can be concealed.
Ukraine is bi-lingual and bi-cultural and that's why I think the only real solution is Federalization. What? Individual regions of the Ukraine might secede from the Union? I doubt that. US states also have the right to secede but the central government does have the right to block secession.
As for Crimea, it never legally seceded from Russia. There was no referendum or vote by the people to do that.