Skripal was a double-agent who used to work for the Russian secret service GRU but defected to the UK intelligence service MI6.
He was arrested by the Russians in 2004.
In 2010, he was officially pardoned and exiled to the UK as part of a prisoners exchange.
So he is unlikely to still possess any not yet revealed intelligence which still has any value.
Still, assassinating him sends a clear message to any other would-be defectors in the Russian secret services: "Do not defect!
Other countries can not protect you from us.
Even if you somehow manage to get an official pardon, we will still be out there to get you.
And we will also get your family."
politics.stackexchange.com/users/3135/Philipp
In addition to @Philipp's answer: Poisoned Russian spy Sergei Skripal was close to {an unnamed} consultant who was linked to the Trump dossier —
The Telegraph
If the above allegation is true, it could be that Skripal was somehow related to the Collusion, and the poisoning of him could be just a revenge for his betrayal.
Since the recent events are developing rapidly, we probably can't answer a single reason .
It can also be a combination of several factors.
politics.stackexchange.com/users/2984/bytebuster for Long Usernames
I have the feeling Russia has more to lose than to gain.
After deploying an obvious trace by using a chemical weapon, it faces penalties by the EU, approved by the USA and further separation from the west (which the USA pushed since years).
On the "what Russia gains" side we have a demonstration of power, by elimination of an insignificant spy.
Maybe there is also some deterrence in case other spies decide to change the sides.
However, deterrence can be achieved with more subtile ways too.
politics.stackexchange.com/users/None/
While all the secrets he knew about would already have been transferred in 2010, he would still pose a potential danger via his personal relationships with his former colleagues and their family members and friends.
While this is pure speculation, it is possible that Mi6 had tried to use Skripal to recruit new spies and that the Russians had found that out.
politics.stackexchange.com/users/4235/Count Iblis
Russia gains nothing.
Whatever knowledge the spy had, has been transferred by now.
"Sending a message to potential defectors" is not needed at all.
A potential defector would be well informed enough - just by being a spy - to know that no place in the world is safe (the Mossad proved that long ago when going after Nazis all over the world).
The fact that many defectors don't even try to hide their whereabouts after a couple of years is evidence that defectors know they will never be safe.
On the other hand, various anti-Russia groups gain something by this poisoning: being able to point fingers at Russia.
politics.stackexchange.com/users/8912/Sjoerd
What would the Kremlin stand to gain from assassinating the ex-spy  Sergey Skripal , now critically ill after being  poisoned on  March 4th 2018  with a nerve agent ?
What would the Kremlin stand to gain from killing the ex-spy Sergey Skripal?
politics.stackexchange.com
<russian-federation><espionage><assassinations><gru>
politics.stackexchange.com/questions/28507
politics.stackexchange.com/users/19963/Vasya Milovidov
Russia may have more to lose than to gain. Russia will have more to lose than gain.
Russia will have more to lose than gain.