Lot's of things can be said.
If a person is defending his territory, his ethic homeland you can say one thing. He is thought of as a hero.
If he was invading someone else's space he is an invader.
If you look at Volyn it is Ukrainian ethnic territory. The Ukrainian Insurgent Army was defending its territory from Polish agression.
In Slovakia UPA did not opperate. or possibly it was one of those Ruthenian groups.
In my particular case on my mother's side the Poles murdered 2 of my grand uncles. The Russians in the Soviet Army 1. You get kicked around by both sides you fight back.
The Poles used discriminitory policies toward Ukrainians in almost every sphere of life. Ukrainians were only allowed to get 3 years of education whereas Poles would get the obligatory 8. The Ukrainians tried to counter this by repeating years in school. The Poles closed the Classes at the Lviv University established during Austro Hungary so the Ukrainians went to Prague and set up a University, Pedagogical college and Politechnic there.
The lists of Polish attrocities towards Ukrainians is also long.
Are we going to spend time seeing whose list is longest. Its a pissing contest. The one thing I do know is that UPA didn't operate on foreign soil - it could barely operate on Ukrainian soil.
Regarding Slovaks there was no reason to be in conflict with them at all. Hungarians possibly. Communists and various Jewish political adminitrators yes but Slovaks no.
Romanians possibly - but they didn't function there.