The motive attributed to the poisonings basically sounds like, "They spoke out against Putin and/or Russia, so Putin had them poisoned." The thing that bothers me about this motive, is that, of all the poisonings attributed to Russia and/or Putin, Navalny is the only person who had/has any reasonable amount of people to support and listen to what he says. Litvinenko, Skripal had no real auditory that would/could make them dangerous, until they became martyred by western media.
I mean, maybe the first poisoning was a mistake, maybe analytics and/or advisors thought that no one would care, and it would all go over quietly, but that only explains one incident, but what about the rest? Even so, I highly doubt that anyone at all cared enough about these people to dedicate time and resources to developing and executing a plan to liquidate them.
So the question is - how valid is the motive to kill some no-name who can reach out to no more than a couple hundred dissidents, if that just makes them a martyr and gives them more publicity postmortem?