Ukraine put it in their post-Maidan constitution that they had to join NATO. Sure, Ukraine’s civil war was its own internal affair, but the main issue at play was hostility to Russia and wanting to join the West. Why would Russia see this as anything but a threat? More NATO bases, another NATO member, and this one adjacent to the Russian heartland. A lot of people scoffed about US bases surrounding Iran, but it turns out that those were there for a reason - attacking Iran! In retrospect, Russia’s position looks more rational rather than less.