It is hard to grasp because it is a stupid theory. If they wanted him dead, they had him in prison and could have killed him. They thought he was more valuable free after a spy swap.

If they spy swap then poison him, next time they won't get a spy swap - nobody is going to spy swap with a country that then assassinates the swapped spy. They will have gained nothing and lost an option. They will be worse off.

It is possible that the Russians are stupid, and are happy to be worse off having made a point. Stranger things have happened. But it is also possible, and indeed quite likely, they are not stupid and there is something else going on.

I think it’s because US / UK security services are just as ruthless and bloody, they just cover it up better. I doubt anyone here disputes the extreme moral failure of Russian state security. It’s just that we should be much more skeptical of countries that are just as bloody and flippant in their lack of concern for human life or decency, like US & UK.