There’s nothing to suggest the Skripals were a liability to the UK and you’d be getting in to quite a grand conspiracy there to just base it on an unanswered question.
DaveFagan excels here at DP in conspiracy theory, anti-Western rhetoric, and Russian apologia.
The UK had no beef with Sergei Skripal. After all, he was a double-agent and worked for MI6 (1995-2004). He was eventually arrested in Russia (2004), convicted of treason (2006), and sentenced (2006) to 13 years in a maximum security prison. In 2010, Skripal and three other Russians were swapped-out in an exchange of ten Russian espionage agents imprisoned in the United States (one of them being the notorious fashion model and spy, Anna Vasilyevna Chapman). Sergei Skripal and his daughter Yulia were granted residency in the UK. They lived quiet, peaceful lives until someone tried to assassinate them with the Russian nerve-agent Novichok on 4 March 2018. A British Sergeant Investigator was also poisoned accidentally. All survived. Not because there is an antidote, but only due to sheer luck and the diligence of the doctors and nurses attending them. Another Skripal daughter, Viktoria, still lives in Russia and had asked her father and sister to return. They had refused.
The UK had no obvious motive to assassinate Sergei Skripal. If the UK government had tired of him, the Skripal's could have been deported back to Russia. One must also be cognizant of the fact that this method of assassination released an extremely lethal nerve-agent in the midst of populated areas. Indeed, a police investigator was also seriously poisoned on 4 March at the Skripal home. This assassination attempt containment/decontamination cost the UK government a minimum of $10 million dollars with over 40 different agencies involved. The Novichok used was weaponized by a state, as lethal as Sarin, very difficult to detect, and exceedingly persistent. It make no sense whatsoever for UK authorities to unleash such a potent killing agent within the UK population. The Skripals were poisoned in early March. This new Novichok poisoning occurred on 29/30 June. Two additional people have now been felled and one has already died. The Novichok agent employed on 4 March has a released-persistence of at least 4 months. That is extraordinary. Even if Porton Down had been able to synthesize such a lethal strain of Novichok (never reported as doing so by the OPCW), releasing it in the UK population was unnecessary (an "accident" would have been optimal) and highly dangerous and damaging to the UK government if such a story ever leaked out.
Russia did have motive (vengeance), UK assassination experience using an exotic poison - Alexander Litvinenko), and the greatest research/production history (Shikhany/Yasenevo/Sosenskoye) with Novichok. Two Russians (unidentified publicly) left the UK via airliner on the night of March 4. What seems to have occurred is the March 4 assassins accidentally contaminated something either before the assassination attempt, or they discarded the Novichok container (it would probably be small cylinder) after the dirty Skripal deed was done. After all, why take a second chance of airport detection on exit from the UK? Moscow has a long and lurid history of murdering journalists, activists, and what they consider traitors. On 7 July 2018, another Russian dissident who was suing the Russian state airline Aeroflot (he was to be awarded £5 million) was found strangled to death in London on his court date.
To put it bluntly and succinctly, the Kremlin had motive, opportunity, vast experience with the exotic killing agent applied, and a long history of domestic/foreign assassination activity.
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