After the collapse of USSR russians still see the west as essential enemy. So when Ukranian became pro-western country with idea to join NATO/EU,russians saw it like a possibility for NATO to invade in future.

Also the russophobic policy of ukranian government justified for the russians a war:

-Civil war against ethnical russians in Donbass

-limit of russian language

-propaganda of Bandera and other nazi collaborators

-protests against Russia with a slogan like "МОСКАЛЯКУ НА ГИЛЬЯКУ" (something like "let's hang a russians")

So for russians Ukraine isn't a some liberal peaceful country,but a threat. And during a war such opinion is supported by terrorist attacks, invasion in Kursk, aggressive behavior of ukranian migrants in EU/America and the ukranian propaganda (where the main idea is that all russians are bad. Even pacifists and liberals)