Putin did it because he's an abusive dickwad who thinks it's his god-given right to control Ukraine. That's it. That's the entire explanation.
People keep trying to rationalize his actions, come up with some more respectable reason, because there's no way this much death and suffering could be happening because one guy is an entitled, controlling piece of shit, right?
But in reality, that's how fascism always works. It's always been how it works. Why'd the Nazis invade Europe? Because they were entitled, controlling pieces of shit. Why'd the Soviet Union refuse to let the countries they liberated after WWII go? Because they were entitled, controlling pieces of shit. Why does China want to annex Taiwan? Because Xi's an entitled, controlling piece of shit.