How is the motive "not there"?

Skripal was seen as a traitor, having acted as a double-agent for British Intelligence. There's a clear pattern of Putin going after "traitors" - Alexander Litvinenko had publicly accused Putin of conducting assassinations and organizing false terrorist attacks before fleeing to the UK and famously being assassinated with Polonium there. Maksim Kuzminov was just assassinated in Spain a few months ago for having defected to Ukraine with a helicopter.

As the article states the arms which were destroyed were likely to end up being received by the Free Syrian Army, in opposition to the Assad regime supported by Putin - or alternatively, to Ukraine, against which Russia had just started a conflict.