Sure there are other perspectives and possible motivations for his actions. But it's the idea that he may not care any more and might do anything that I find scary. If he's willing to risk full scale invasion across all of Ukraine after the international focus that preceded it, would he also be willing to launch supersonic nuclear weapons? In his televised statement he said Russia is ready for all outcomes.
For the USA what probably worries me most is cyberattacks. The USA is such a soft target. Our government can't even secure it's systems after a decade of laws, warnings, bad reports, and actual attacks.
I do hope I'm wrong and Putin is actually sane and has a strategic plan that makes sense for his country.
Putin is at the top of the pyramid but he is not the sole ruler of the Russia. Its not like if he dies Russia will magically become exemplary democracy.
There were and still are people lining up and aiming to take his seat, but as long as Putin appears to be strong and gives his underlings enough he will remain on the throne.
"Take all your overgrown infants away somewhere And build them a home, a little place of their own. The Fletcher Memorial Home for Incurable Tyrants and Kings. ... Boom boom, bang bang, lie down you're dead."
--- The Fletcher Memorial Home - Pink Floyd