Putin is honestly mentally challenged or something.
He wants to be a czar
You could just let ukraine be
Ukraine was moving closer to the EU after the protests and election in 2014
https://en.wikipedia.org/wiki/Revolution_of_Dignity
Russia invaded eastern Ukraine and Crimea shortly after.
Russia and Ukraine share a common ethnic background with intermarriage and thousands of people with relatives in the other country. Your relatives in Ukraine tell you how great freedom, democracy, and the European Union are You still live in Russia, won't you want the same for your own country?
Putin can't have that happen because that would be a threat to his power. So invade Ukraine to stop it and set up a puppet government. Only this time Ukraine was ready and has fought back hard.