Just Because Something's Irrational Doesn't Mean it Didn't Happen

Thing One:

It would have been completely irrational for Vladimir Putin (or one of his underlings in Russian intelligence) to order the assassination of Sergei Skripal using novichok.

For one thing, Skripal was part of a sort of protected class: A spy for the west who was caught, tried, and sentenced to prison in Russia, then exchanged for Russian spies who had been caught, tried, and sentenced to prison in the US. The thing about prison exchanges is that if one of the exchanging parties comes back and starts killing the spies they exchanged, such exchanges will grind to a halt.

Which means any such killing would not have been for the purpose of "sending a message," since that's not a message Putin would want to send. Which in turn means that even if the Russian state was behind the attack, it wouldn't have been carried out using a weapon that would make everyone say "ah, the Russians must have done it." The attack would have been made to look like an accident, or a burglary gone wrong, or whatever.

Thing Two:

It would be completely irrational for Bashar al-Assad to piss off the world with a chemical attack every time his regime was having some success putting down the rebellion in Syria.

For one thing, each of the chemical attacks he's been accused of has occurred after a significant line of military success has been crossed, not at a time when such an attack might have made a material difference to the outcome.

For another, he can't help but know by now that such attacks are treated as a "red line" by his enemies from abroad and are likely to result in attacks on his own forces which would not otherwise have happened.

So, extreme skepticism is warranted when assessing claims that the Putin regime had Skripal attacked with novichok and that the Assad regime had Douma attacked with chlorine.

BUT!

Just because something is completely irrational, that doesn't mean a government generally, or a particular government employee, wouldn't do it. Governments and their employees do irrational crap all the time.