In one sense there is nothing mysterious about the death of Dimitriy Zelanov at all. A Russian oligarch living in exile in France, he breached one of President Putin's unwritten rules for Russian businessmen living abroad, and he fell down the stairs in his French villa, unsurprisingly breaking his neck.
There is nothing unusual about this. The Kremlin does it all the time to Russians living abroad who President Putin deems to be traitors. Such assassinations are always ordered by the Russian President personally, and are typically executed by the GRU, an elite Russian military intelligence unit trained inter alia in how to conduct assassinations abroad. So there is nothing strange about any of this (apart from in the broader sense, in which all things Russian are incredibly strange).
Nevertheless take a closer look:
Mr Zelenov held both Russian and Ukrainian passports (theoretically illegal, but nevertheless).
His official status in France was that of a Ukrainian refugee, albeit it that he lived in a large mansion in a French suburb of Monaco, which almost seamlessly slides into Monagesque territory. This area is very expensive and there are or were several monied Russians living there. They want to be close to Monaco, where they do their banking; but Monaco itself is unsuitable for them as virtually all of Monaco's apartments (there are almost no houses per se in Monaco due to high population density amidst the side of a mountain that drives real estate prices sky high) are extremely small.
Monaco is a quite ghastly place to live, for lots of reasons. Aside from tiny apartments for outrageous prices, the roads are sharp, hilly and bendy; the hotels are dreadful by international standards yet extremely expensive; the city is awash with Eastern European prostitutes (there are hordes of them in the hotel lobbies, casinos and bars, 24 hours a day) and the government is very interfering in private business, coming to check up on you periodically on pain of withdrawal of your corporate licence. Government officials are almost Stalinist in their refusal to explain the civil service system in any intelligible way. Hence the monied Russians live just across the border in France, where space is more ample, driving is easier and there is a more relaxed air. And French bureaucracy, although sometimes tedious, works quite well.
You can actually walk from France to Monaco to Italy (on the western side of Monaco) without any checks of any kind. The same is true of Monaco's principal public transport artery, a local train that runs from Nice in France to Ventimiglia in Italy, via downtown Monaco.
Monaco makes itself very easy to get in and out of without any questions, if you are very rich.
Mr Zelenov earned his fortune as the Chief Executive of an entity called 'Don-Stroi'. Although this purports to be one of Russia's largest commercial construction companies, it is nothing of the sort. It is run out a residential apartment in an obscure part of Moscow.
What Don-Stroi really is is a front for a Kremlin money laundering operation into the west. So (for example) Don-Stroi makes subcontractor contracts with offshore construction companies (probably all really owned by and run from the Kremlin or some suitable Moscow lawyer's office). In this way, Don-Stroi gets to pay sums of money abroad - outside Russia - which appear clean because the money arrives in the offshore account associated with the offshore contractor pursant to apparently legitimate work done for an apparently legitimate Russian commercial construction company.
Presumably Mr Zelenov's sin was revealing this structure to the French authorities (or seeking or threatening to do so), which would have the effect of potentially freezing offshore accounts owned by the Kremlin pending some sort of legal investigation in a western European country.
The method of assassination used - throwing someone down the stairs (they have probably already been slowly beaten to death before a dead body is thrown down the stairs) is in a category of assassination methods used by Russian intelligence agencies against people who are particularly despised or loathed or the Kremlin wants to make a sharp example of - their death was slow and painful. A person threatening to reveal the Russian President's banking secrets would merit so barbaric a punishment.
As to the GRU assassins' route, we suspect it was Moscow-Istanbul-Lugano-drive to Monaco and then the assassins just walked into France with wooden sticks or some similar such object. Then you execute the assassination, throw the assassination tools into the Mediterranean and retrace your steps. In all likelihood foreign passports were used (Hungarian or Serbian are two examples of GRU foreign-held passports.)
We imagine that the Monagesque police may have facilitated the operation; Monaco's Prince Albert II is known to be personal friends with Vladimir Putin and his 'country' operates as a crummy but ruthless autocracy led by him and with no dissent brooked. The Monagesque police are certainly capable of participation in such crimes; they may have helped locate the victim through access to SIM card triangulation data that they share with the French security forces.
Recall that the Monagesque police have a reputation as a bunch of violent thugs in expensive uniforms; it is understand that they routinely beat detainees; and Monaco prison by all accounts is a very dangerous place. Recall also that Monaco has strict censorship of all materials published not in the interests of Prince Albert II or his ruling clique. Censorship laws are enforced with Police intimidation and violence and the peremptory withdrawal of residence permits. Moscow has no functional judiciary to speak of in the western European sense. In many ways, Monaco is run akin to a mini-Moscow by the sea.
We speculate that a condition of Mr Zelanov's refugee status in France (a long-term Kremlin insider is a curious candidate for refugee status at the time of writing amidst a war of aggression pursued by Russia against Ukraine and which has caused many people associated with the Kremlin to be placed on pan-European sanctions lists) was that he spill the dirt to the French.
Alas, the French were not on the ball in protecting Mr Zelanov from a risk of assassination, which they probably never anticipated in a virtually crime free, easy going neighbourhood of the Cote d'Azure.
This was a very unpleasant way to die, and the Kremlin clearly wanted to make sure that everyone understands the consequences of financial betrayal of the interests of the Russian government's highest echelons.
