The Salisbury attack was very much a " near act of war" against the United Kingdom. In the foregoing, had Russia just wished to murder an ex agent, then to use someone with a gun or similar would have been much more straightforward and effective. Putin and those that surround him wished to make a statement of their power and following that, Russia's immunity from any reciprocal action following their "assault" on Britain.
Putin had every reason to believe that the above immunity would be the case as Russia has "got away" with the downing of an airliner over the Ukraine, the invasion of Crimea and destabilising of former Soviet bloc countries surrounding the Russian Federation. Therefore, a further demonstration of Russia's resurgence of power it was felt (I believe) could be carried out against a somewhat isolated United Kingdom.
In the above, the Russian Government has misjudged the support that Britain has received from NATO allies doubtless after the UK shared all the close security information in regard to this attack with those allies. Why has not the UK government given Russia a sample of the nerve agent recovered from the scene as they demand, then obviously for the same security reasons.
The nerve agent used was designed to be undetectable and in that battlefield medical teams and others treating those affected by the agent would not know what they were dealing with and therefore how to treat. Therefore, any nerve agent recovered from the scene may also show evidence of how successful detection was carried out. In that, to supply a sample of the recovered agent to the Russia for analysis may also give them information on how it was detected.
The Russian government denial today (15/03/18) that they never ever developed this nerve agent only gives credence to the above. In that, several years ago two scientists who worked to bring forward this nerve agent defected to the United States and gave information on this agent that could only have been gained by someone working deep within its development.