If Litvinenko committed crimes, then Russia should have asked Britain to extradite him so it could put him on trial. Russia did have options other than assassination. That is not possible with terrorists who are hiding out in the mountains in Yemen. You are using the time-tested leftist technique of declaring moral equivalence to excuse the Putin regime’s actions.
We could have asked Yemen to turn over the targets, but political they would have not been able to. So we blew up his house.
Russia did ask for him, Britain said no. Again, Litvinenko was connected to terrorists. So they killed him.
The big difference is we asked Yemen before we blew up the house.
IMO, both are hard to justify.