Probably thought there wasn't any choice of an ally who would confront the SU. The USA didn't. No one else did. Of course, most of the sources nowadays are maintained by Russia, USA etc., soviet/communist sources so you will only get one side.
In Russia, certain cronies and clans turn against each other when interests no longer coincide. Is it possible that this happened then? I read that Bandera wanted a 'Greater Ukraine' and when Russia/Novorossiya decide to occupy territory, no one bats an eyelash. But, because these aren't neo-Soviet types, they are villains. It's a matter of perspective or point of view and those nationalists probably thought they wanted to secure their own territory. There were a lot of territorial disputes then and various nationalities/ethnicities all living in the same area.