Ukraine is scary to him because Ukraine has shown that (despite their issues w/corruption) they have what it takes to be a Western style democracy. If Ukraine can succeed and be properous and free - then Russians may want the same thing - which could spell the end to Putin and his oligarchs.

The West is making a grave mistake in appeasing Putin (and Xi, but China is heavily dependent on Western consumers in a way Russia is not) who only respects military force. NATO should've built up forces in that region once Putin moved on Crimea. By doing little in response Putin has become emboldened. He messed with our elections in 2016 and helped to install Trump. Putin has never paid any real price for these actions and thus continues on.