Both acts were assassinations. Litvinenko was more than a common criminal. Less (maybe) than the guy in Yemen, but he was tied to a group that was funding Russia's terrorists in Chechnya.
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.