Saddam wasn't assassinated. He was tried by a tribunal established under Iraqi law and executed after that trial found him guilty.
Hitler killed himself rather than face capture, trial and execution.
Gaddafi's death is disputed at best, but happened during open hostilities and was likely at the hands of Libyans.
There are a few reasons not to do this sort of thing:
First, we cant say we want a rules-based international system, but then break those rules when it suits us. You may as well just throw the whole thing out the window.
Second, Putin is not an island. He still enjoys massive support, though it seems to be reducing thanks to the war. Make him a martyr murdered by the West and then what? His supporters will enjoy a massive mandate and justification to follow in his footsteps. It's really important to remember that there is an honest belief among many Russians that the West wants to destroy Russia, which is why so many Western actions are seen by Russians as a direct threat.
Third, the overall history of Western-driven regime change is hardly a happy one. The West couldn't kill Castro, how do you think it would go against a famously paranoid and well-protected president with significant resources? And what would happen after it failed?