If Russia behaved rationally they would have made a deal with Ukraine (decades ago) that protected the rights of the Russian minorities in Ukraine and continued massively profiting from gas and oil exports into Europe.
Russia did, many times, last time in 2010.[0]
But the US and the EU didn't like it, so the coup in 2014 happened with American help[1] which overthrew democratically elected president of Ukraine who introduced the law protecting Russian speaking Ukrainian citizens.[2]
The law was repelled right after the coup and Crimeans supported the reunion with Russia.
[0] https://en.wikipedia.org/wiki/Russia%E2%80%93Ukraine_gas_dis...
[1] https://www.bbc.com/news/world-europe-26079957
[2] https://en.wikipedia.org/wiki/Language_policy_in_Ukraine#201...
Parliament votes 328-0 to impeach Yanukovych on Feb. 22; sets May 25 for new election Parliament did not vote to impeach the president, which would have involved formally charging Yanukovych with a crime, a review of the charge by the Constitutional Court, and a three-fourths majority vote in parliament—at least 338 votes in favor.[0]
[0] https://en.wikipedia.org/wiki/Revolution_of_Dignity#Removal_...