Russia and Ukraine have a very long, complicated history, like 1100 years worth. I am by no means a Russian expert, just someone who has been fascinated by Russian history and read a good deal of books on it's history. Both Ukrainian and Russian's lineage traces back to the Rus, and the Kievian Rus of the 900's. The Russian Orthodox Church was founded in Kiev, in the late 10th century. The Mongal invasion separated Kievian Rus and Muscovy for about 250 years. During that time Moscow and it's influence over the Slavs grew, and Kiev's fell. In the 1600 the Cossacks moved away from Poland and swore allegiance to the Tsar, which effectively brought what we know as the Ukraine into the Russian Empire until the end of WW1. 1922, Ukraine was a founding member of the USSR where it remained until 1991. The Russian have considered the Ukrainians to be ethnic Russians "Little Russians", due to language, religion, culture for a long time. Despite the close and historical ties, most Ukrainians consider themselves Ukrainian, aside from swaths along the Russian boarder - Donbass (hello Don Cossacks) etc.
Peter the Great was obsessed with taking Azov from the Ottomans, to give Russian an ice-free port on the Sea of Azov, and thereby the Black Sea, which is dominated on the northern side by the Crimea. The Azov region is still the only ice-free port that gives Russia access to the world that doesn't require going thru the Artic, or the Pacific, and the Russian Navy has been intent on dominating the Black Sea since the early 1700's. Then there is the famine in the early 30's when Stalin starved 3+ million Ukrainians - that Ukraine recognizes as a genocide against Ukrainians at the hands of the Soviet Union. The relationship is complicated, but there is some validity to the Russia feeling Ukraine is culturally important to Russia. Not unlike Kosovo to Serbia, but that's for a different day. chy
Now you have a Cold War era KGB spook in charge of Russia, who is nuts and still pissed about the dissolution of the USSR and hyper nationalistic. In his mind, it's part of Russia, and having it be part of NATO moves the Iron Curtain from Eastern Europe, puts it on his doorstep, as well as puts the most wide open indefensible part of the Russian territory on a NATO border. Oh, and Ukraine has enough resources, that if tapped, could rival Russia as a petrol state.
I would agree with the Russian/Strongman leader thing, they have been "governed" by an autocrat since Ivan The Terrible in the 1500's. There was a symbiotic relationship with the Tsars and the boyars that keep each in power/rich and the people had zero say in anything. It was replaced by the Soviet General Secretary and the Party Hierarchy, and now it's Putin and the Oligarchs. It simply has never changed for the Russian people. Democracy in Russia lasted from about 1993 until Putin took office around 2000. So one thousand years of history, with less than 7 years of a democracy. It's a foreign concept by and large to the Russian conscious. Which in no way means the "deserve" to be oppressed or brutalized - nor does Ukraine deserve to be invaded in a war of civilian annihilation.