Just a thought, and I don't know if this has been voiced elsewhere. And this, I freely say, is speculation. The nerve agent used seems to have been remarkably ineffectual - you expect these things to kill within seconds, not leave people lingering on for months. Received wisdom - and such military training as I got in these things stressed it - is that the merest droplet on the skin kills. Within minutes, at the outside. It's almost as if the variant used in Salisbury was denatured or diluted, somehow. I'm wondering here. From the Russian point of view, knocking off a dangerous dissident was a result. But what if the real reason was something else: getting the gobby dissident who picked the wrong side was a bonus. What if. somebody was conducting a practical test. Of the ability of a possible enemy to respond to a chemical attack, in a town in one of the most advanced societies in the world. Just one chemical attack tied up the civil defences of Salisbury for months and drew in expertise from the rest of Britain. News coverage enabled the Russians to practically assess how much of Britain's resources would be tied up in investigating and cleaning up afterwards. And this wasn't a full scale bombing or missile attack - just one small isolated incidence which affected a handful of people. Scale up from that and imagine a single aircraft dispersing the full-potency version from overhead as an aerosol... a remarkably cheap way of assessing how open to attack a possible enemy is and how it can be tied down...
Interesting bit of lateral thinking. Whatever the motives, elimination , cock up, or testing, Putin's mob must have been 100% sure that the finger would have been pointed at them. I suspect also that a full scale chemical attack would elicit a somewhat different response. But never the less a fascinating idea AgProv.