They just do.

For them, every Ukrainian not welcoming Russian tanks and missiles with flowers is a nazi.

On July 12, 2021, Putin published an essay explaining that Ukraine has no right to exist and should be a part of Russia. The link to the Kremlin website is here: http://en.kremlin.ru/events/president/news/66181. It does not work now. The link to the web archive is here: https://web.archive.org/web/20220331143846/http://en.kremlin.ru/events/president/news/66181.

If you seek a better understanding of what is going on, please check my seven articles on this subject.