There is no Russian motive. http://www.informationclearinghouse.info/48963.htm
The motive lies in the West. It is the latest orchestration in the ongoing demonization of Russia. The demonization is a huge boost to the power and profit of the military/security complex and prevents President Trump from normalizing relations. The military/security's budget and power require a major enemy, and Russia is the designated enemy and will not be allowed to escape that assigned role.
The false accusations against Russia are damaging the Western countries that make and support the accusations. There has never any evidence provided for any of the accusations. Consider them: the Malaysian airliner, Crimea, the polonium poisoning of a Russian in the UK, Putin's alleged intention to restore the Soviet Empire, Russiagate and the stealing of the US presidential election, other charges of election theft or interference. The current Skripal poisoning. Accusations abound, but never any evidence. Eventually even insouciant Western peoples begin to wonder about the transformation of evidence-free accusations into truth.