Another nice article about why Russia don't want a Russia-heavy anti-kleptocracy country on their borders.
[Russia's] worst nightmare is a Ukraine growing into an alternative Russia - a better place for talented and entrepreneurial Russians to live and work than Russia proper. Or even worse - a united pro-democracy front rising against the mafia state in all of the former Soviet republics, with Ukraine in the lead.
The West misinterprets Putin by thinking that he wants to rebuild an empire - it is much more likely that the real goal is a compact, monoethnic and ultra-nationalist Ukraine, with the Russian element reduced to a minimum. Being intimately linked to Russia at many different levels, a successful, democratic and tolerant Ukraine threatens the existence of the mafia state and that's what is at stake for the Kremlin rulers.