It is moral for countries to provide means to protect their own country. Whether or not a former KGB spy was poisoned is irrelevant. Countries need their own military, as well as their own spy network. However, a lot of times those spy networks do immoral things, but that doesn't make the job itself immoral.
KGB spies contributed very important roles during the cold war. However, from moral perspective, KGB spies did many bad and terrible things to the other side, which is the United States and the US citizens. The spies stole so many precious information, documents, technology, and also they killed a lot of people in the US. So, any jobs which kills people and stealing things are not morally good at all.
I think the implications of morality in the spying of nations is a really subjective argument. It is hard to decide what is moral or not when the laws are broken on both sides by the individual spies, they are acting as free agents in a lot of circumstances and cannot be trusted.
Spies get information. This information includes, but is not limited to, monetary movement, military movement, and other spy movement. These secrets can cost lives, either in the getting of them, or the telling of them. At the point when a secret spills any blood, that is when there is no morality at all.