I think a good number of you are missing the point.
The first thing Putin was trying to do was the put the fear of God up us simple minded folks in the UK, and test the International response to see if our so called Allies were willing to sacrifice their own interests to stand by us.
That Putin did this through the medium of attacking a British Double Agent was incidental but calculated too. Because Putin was sending an unambiguous terror message to any other Russian who might put their interests above those of Mother Russia, as well as writing a Russian signature on the event to ensure the UK and the rest of the world didn't miss the point.
Because the target was a British Double Agent means you can discard any other fanciful actor as the perpetrator - the Russian Mafia have no interest in a Spy, the Arab Factions, if they were interested in making a point, would firstly, have great difficult identifying the Spy, and, even if they could identify the Spy, would see it as confusing their message. If Arab Factions wanted to make a point with a Russian Developed nerve agent, they'd have done it in Central London, and not with a retired spy living in Salisbury.
Putin is less interested in what the UK does in terms of retaliation, he will be monitoring the sacrifices the Germans, French, Dutch, Danish, and Americans are willing to make to stick up for us.
I suspect he calculates Trump won't want to stop Russians killing Muslim Fanatics in Syria or Chechnya, and in any case Trump'll be too busy fending off political and personal attacks from his own countrymen in the US.
Putin is neither mad nor idiotic, simply very very dangerous. If the west fails this test, the next test will be the sending special forces into Estonia, Latvia, Lithuania, or Poland and kidnapping the odd Pommie or two. Or perhaps he'll simply annex 50% of the landmass of Estonia, Latvia, and Lithuania - the places where ethnic Russians are in the majority. Why not take what you want when no one will lift a finger to stop you.
Putin wants clarity - he wants everyone and his dog to know he did it and he can do it on this occasion and the next with impunity. The message from Vlad needed to be clear, that was the whole POINT.
Given the reasoning seen on here, I'm surprised no-one has yet named Nicola Sturgeon as Ms Big, all the suggestions I've read so far are just as fanciful.