They could do it, it would absolutely destroy our geopolitical relations with important countries in the process, which is hopefully why they didn't try.

? An ex KGB guy was assassinated in London with radioactive poison and lived to name the killers.

That ruffled the feathers until a few years later when it was all swept under the carpet.

Now the disagreement is over Ukraine and Syria. So Russia is in the bad books again for the killing

The UK isn't a major superpower though. Russia sees the UK as the poodle of the United States, and although it loves it's poodle, poodles can easily be replaced. Putin was sending a very specific and threatening message to any Russian dissidents living in the UK. You aren't safe even if you thought you were. Shut up.

Haha, best characterization - "loves its poodle, but"

Hardly swept under the carpet. It's an ongoing diplomatic headache. Just a couple of weeks ago, a public enquiry (which Litvinenko's widow had pushed for) publicly announced that Litvinenko was most probably murdered by the FSB with Putin's approval.