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I think the unhinged rants are crafted to scare the West and his parliament, and they certainly work.
They certainly work for me. The theory of nuclear deterence, once known as MAD or mutually assured destruction, assumes that those in charge of the nukes are sane; if they're insane they won't reliably be deterred.
I hope we've all seen the film Doctor Strangelove:
https://en.wikipedia.org/wiki/Dr._Strangelove
Happily, this story never came to pass, but there are three characters who seem to be bonkers: Strangelove himself, General Jack D. Ripper, who thinks the communists are polluting his bodily fluids, and the bomber pilot who sits on his own bomb as it is released.
If you've watched this you won't want to play nuclear chicken with madmen.
As for assassination, it has been pointed out that Russia has some similarity with Rome (no effective constitution) . It was fairly common for Roman Emperors to be killed by their praetorian guard - obvously the guards were very well paid, but even so there comes a point when they decide their boss is off his rocker and it would be in their own interest to have someone saner.
The problem with replacing him is that he's killed or jailed or exiled his opposition and he's had 20 years to fill his administration with obedient lackeys and old mates and vicious thugs. A real regime change would take a massive public uprising. If the police/security services/army are loyal to him, he would kill thousands of the public as 'terrorists'.
He does fear assassination. Here's a tabloid account of his security. https://nypost.com/2022/03/04/heres-how-putin-protects-himself-from-assassins-and-coups/
As for motivation, I think he really believes he can restore Russia to somewhere near the status it had in Soviet times. Living standards have gone up a lot under his rule, but the influence and prestige and respect have shrunk. He wants to go down in history as a national hero. I think the unhinged rants are crafted to scare the West and his parliament, and they certainly work.