Well, the thing about Putin is that he considers himself to be responsible for the well being of the Russian people. And I do not mean just Russians in the Russian Federation. The active acts of violence against Russian ethnic or Russian speaking people in Ukraine are what led to this result. Which is why had Ukraine gone with Minsk or Minsk II it would have likely ended there.
When Putin says the fall of the Soviet Union was a tragedy, they typically cut the quote there. But the quote goes further. A lot of people found themselves locked into nations which they did not consider as their own when the Union fell. Remember that in the Soviet Union you were given a position, and then you were given housing according to your position. So a lot of people who were not actually native to a given place ended up there. You were born in one place and live there until high school, then you might have studied somewhere else, and went to work somewhere different altogether. A lot of Russians were technical experts so they were spread all over the place. Many ended up in Ukraine, or Kazakhstan, and there are substantial parts of them in the Baltics. Some came back but others did not. The sick thing is the US knows this and fuels the ethnic tensions already present in those places because of this as a way to weaken Russia. A lot of those countries try to force these Russian ethnic populations to assimilate into speaking their own language or even worse. So it is kind of complicated.
But this can be a problem. Hitler also started a lot of wars before WW2 with a similar argument. And you know what, a lot of people were actually in favor of him back then, for example Gandhi was a supporter of Hitler. Gandhi thought he was doing something similar to what they wanted to do with unification of India. But we know how that turned out. I think Putin should have just facilitated migration of Russians into Russia and left it at that. And he might have. But with NATO growing close to the strategic depths of Russia and placing major conventional and in the future likely nuclear weapons right next to the Russian heartland Russia is left with little choice but to make war to expand its strategic depth. Much like what Stalin did just before WW2.