“well...there is no "allowed"..because there is no authority to decide what is and isn´t allowed.”
Yes there is. It called International Treaties. That is why UK can protest when a Russian is probably poisoned by KGB (under new ID) in London.

“you either have the ability and the power to do something like that without being concerned about the consequences or you don´t.” Well, that is bit tricky. In case of French terrorist protected by UK, the French had the possibility to kill him/them. The French didn’t because UK is a friendly country… Then the limits of the UK policy towards Muslim Extremists/terrorists were exposed then the UK extradited the terrorist(s)… But you are right, of course. Russia doesn’t care of UK opinion, so…

“Somehow this topic makes me think of Pompey the Great: "Don't quote law. We carry swords!" His problems came when Caesar got a bigger sword.

About how would France react in case of terrorism (or even lesser things): We have New Zealanders in this Org who probably remember the Rainbow Warrior? One journalist killed… When Libya was a little bit too much, the French Army launched a raid at Ouaddi Doum (Wadi Dum) and pushed it back. You can find multiple example of this kind of actions…

Now, how I think about terrorism? How to deal with them? No idea, really: Intelligence gathering, infiltration, liquidation when necessary. But it would be better to have the justice at work.
No “friendly” governments harbouring criminals could be a good start…

My problem is not the action, my problem is the moral lecture that we, the West, give to others. If I follow you, nowadays, the Taliban would be legitimated to launch an attack on USA towns. Or the Iraqis… So what about all the South Americans that a lot of them died thanks to the CIA and the “counter-insurgencies” programmes?
The Army can be part of the solution, but not the entire solution. Nor the Politicians are.
The use of violence is legitimated by laws, and has to be questioned by the laws.