I'm no assassin but wonder how someone could fail to kill someone with poison. Aren't there a million deadly choices out there that would do the job reliably? Is the FSB security apparatus this incompetent?
I think its a mixture of incompetence, micromanaging (you need to use very inappropriate radioactive or chemical warfare agents like Novitchok) and indifference (since terror is a major goal).
In the past they used radioactive poisoning which is pretty easy to trace back and caused collateral damage. That's problematic. Poison is very dependent on dosage and delivery without compromising the agent might be tricky.
https://amp.cnn.com/cnn/2020/12/21/europe/russia-navalny-poi...
But could be warning as well, ofc. The point is, we don’t know. Whatever the outcome, it’s a show of force.