Either :
a: Russia has carried out the act
b: Russia sold the nerve agent to someone else and they did it
c: Russia has lost control of a significant quantity of one of the most deadly substances known to man - and then the current owners randomly 'took out' one of Mr Putin's most hated enemies
I'd go with (a), can just about believe (b), but find (c) just about as ridiculous as believing that Mr Putin will win a fair and democratically transparent election next week.