Earlier quoted context omitted.
Which we know, but apparently Putin doesn't. I think in one of his writings or a public speaking engagement he said "They're (Ukrainians) Russian even if they don't know it".
I read an article recently—which I'm afraid I can't find anymore—that posited that part of what's going on is that Putin, and some of the rest of Russia, has a rather surprisingly expansive view of what it means to be "Russian", combined with the belief that if one is, by that definition, Russian, it is good and right to be part of Mother Russia. It also pointed out that by extension, this means that anyone in Ukrain…
From an economic standpoint, or the standpoint that war is bad and should be avoided this obviously looks insane to you and I. But if you have different motivations, such as you and others described, it could look perfectly rational. It's all about the parameter space you operate in that determines your actions.
(1) Ezra Klein has some fantastic podcast episodes about this - let me know if you'd like a suggestion or two