There are two competing theories here.
One, false flag op. Why in the world would the Russians use a nerve gas that would be instantly traced back to them? They had numerous opportunities to kill this spy. They held him in prison. His daughter was just in Russia apparently. If they wanted to kill them, why do it in a park in the UK?
Two, to make an example. They did it fully expecting the method to mark them as the killer. They did it to send a clear signal to anyone else thinking about getting out of line. They see the Brits as such wimps, they had no fear of any retaliation.