The entire charge is ridiculous. There is no way in the world that a professional organization like Russian intelligence is going to order any operative to use any kind of a poison gas to kill an individual target. The 99% chance is that the spy or operative being given such an order would simply pull out his service pistol and shoot the idiot trying to give the order or relay the order to him.
Why is this USA business? It happened on British soil. Economic sanctions are acts of war and have already led to big wars. Too much of this is conjecture and could be deception.