I thought I explained that already.
If a person speaks out very publicly, saying "Putin's guys did it", and you want the public to believe that, because you or your guys actually did it with the objective of making people believe Putin was responsible, what better way to reinforce the widespread conviction that "Putin's guys did it" than by promptly offing the guy who said publicly "Putin's guys did it"? Remember, Litvinenko had also very publicly said Putin's guys were trying to kill him -- next thing you know, he's dead.
I have more than a little trouble believing that Putin and his operatives are stupid enough to repeatedly make themselves look like vicious political assassins by offing prominent people very shortly after they've spoken out publicly to accuse Putin & Co. of being vicious political assassins. I think there's a better than even chance that Putin's guys did NOT do either killing, and that the responsible parties' chief objective is to destabilize Putin's regime for their own gain, by making the world believe he's a ruthless power-gripper who has a worldwide network of operatives who can get away with offing high profile people who credibly speak out against him.
The total effect of these incidents and the public perception of them is to WEAKEN Putin's power in the international political community. I have to wonder why Putin and his guys would want to do that. And I don't really believe that they're such a bunch of bumblers that they are accomplishing that result unintentionally, especially since there is now a pattern of repeating the same power-weakening procedure in an almost identical manner to the previous round.