The play, based on the historic murder, is set in the 12th century, and the knights are freelancers, spurred on by the ambiguous words of their irritated king, Henry II, namely ...'Will no-one rid me of this troublesome priest?'
It's possible the Skripal poisoning case was a rogue or ‘false flag’ affair… either by freelance elements in the FSB (the Russian secret security apparatus), or by some oligarch who wanted Skripal dead … and was acting, like the four knights, in the possibly mistaken view that such an assassination would ingratiate said oligarch to Putin.
|No party drug|
I even think that there is a possibility that the Skripal attack was a CIA / FBI set-up to frame Russia, given the reluctance by Trump to implement sanctions against Russia, and Trump’s open contempt for the FBI / CIA / American intelligence services. ‘OK, if Trump won’t sanction Russia for making fools of us, see if he can ignore this geopolitical storm’.
These are Machiavellian theories, but with some credibility given the Iraq Weapons of Mass Destruction fiasco. The CIA have done stranger things, and they may be the only folk outside Russia capable of deploying the nerve agent Novichok effectively.
However, I still put these theories at a probability of no more than about 25%. Maximum.
Case not proven (yet) on publicly available evidence, but extremely strong.