It almost certainly happens all the time, all over the world. We only get to hear about it when the assassinations are sloppy, like in the case of Litvinenko, or bizarre like in the case of the Skripals'.
Putin bumps off 'traitors'. The problem of traitors is relatively small in the UK because, well, it ain't Russia! The UK has no reason/need reason to kill those who betray the UK. You will note that Putin does not kill foreign nationals, as that is the kind of thing which would necessarily cause other nations to react and react strongly. I am not defending the UK's inaction, but I think that's the reasoning operating here.
Putin has made comments to the effect that traitors are fair game - and that he has respect for adversaries (foreign agents/operatives, I presume). I think it is just pragmatism though - re:
my above comments. He knows how far he can go and get away with it. I also think that it is all just one huge game to him. He's probably a true sociopath.