The real reason I think "leftists" in the west are so pro-ukraine has nothing to do with what's actually going on there. To me it's just an extension of political divisions in the west itself. As I said before, being pro-putin is a form of anti-establishment rebellion in the west, and the flipside of that is that being pro-ukraine is generally the pro-establishment position. This is why there was such a sudden break between the Republican party in America (pro-Ukraine) and so many of their voters (pro-Putin or apathetic). It's also why "leftists", who are for the most part middle class and aligned with the establishment, so quickly went along with the establishment position on Ukraine.
Bear in mind that very few of these people knew a damn thing about Ukraine until this year, especially in the english-speaking west. The suddenness and hysteria of their realignment can only be understood in terms of positions they've already taken in the home country, not the conditions in Ukraine. I think for example, a lot of democrats immediately drew parallels between Trump and Putin, while populists drew parallels between the ukraine crisis and the arab spring or iraq war. This flared up existing passions that come out as pro or anti Putin, while ostensibly having nothing to do with him.