Sanctions could help, but are expensive politically and practically. Giving the Mossad marching orders in Moscow and Syria might be more productive, and deniable. Whatever we do, it will engender a violent response, but the alternative is unacceptable weakness.

Of course, it’s the U.K. being targeted, and they are weak. I somehow doubt that Putin would pull what the US would call a terrorist CBERN attack on US soil.

Allegedly

The police haven't disclosed a cause of death yet. It's premature to say that it was/wasn't foul play. We'll probably know soon enough.