"There are many theories. The most obvious answer is Sunday's presidential election. True, Putin is guaranteed to win. He has scarcely bothered campaigning. But the Kremlin remains worried about turnout, amid widespread voter apathy and calls from Alexei Navalny, Russia's most prominent opposition politician, to boycott the vote. The authorities want to the poll to look authentic, even if it isn't.
Over the next few days, state TV channels will pump out this message: Moscow is again the victim of a western conspiracy.
Russia under siege is a favourite Kremlin theme. Conflicts with the west can bear some fruit: Putin has maintained the bump in his nominal popularity rating after his annexation of Crimea, despite western condemnation and sanctions. The wave of patriotism that followed also split the Russian opposition.
So a row with London can do Putin no harm, especially among voters who share his uncompromising nationalist worldview and his smouldering sense of victimhood.
One former senior Foreign Office adviser said it was a mistake to assume that Skripal's spy work for MI6 triggered the decision to poison him in Salisbury. Skripal was merely the "instrument". The real target was the UK, he said. "I don't think it was about Skripal. It was a geo-political intervention."
The adviser added: "Moscow's goal is to demonstrate the UK's weakness and isolation and to drive a wedge between us and other countries. The Kremlin understands how to make these sorts of interventions at just below the level that will trigger a serious collective reaction against them."
If May fails to react adequately, she would appear weak. If she tries to fight back against Russia, she would discover the limits of collective solidarity, the adviser suggested.
There are other theories. Grigol Chkhartishvili, best known for writing detective novels under the pen name Boris Akunin, suggested Putin was betting on a British retaliation that would drive wealthy and prominent Russians out of London. The community of Russian émigrés (and families of wealthy businessmen and officials) was "one of the weak points of the regime", he wrote, and forcing them out would be "useful and beneficial" for Putin.......
The Skripal attack also appears to have been calculated for its domestic impact. It sends a chilling message to anyone from inside Russia's spy agencies and bureaucracy thinking of cooperating with western intelligence. The message: that the state can mete out punishment at its own pleasure and in the most barbaric way. Oh, and your family might suffer too."