Etudiant, I love your confidence in Russia’s apparatus of murder.
In my experience generally in life so far if there is an option to choose between “conspiracy” and “cock-up” when an operational outcome is sub-standard, then “cock-up” is invariably the reason.
The Russians use poison in cases where they fear blowback and need (scarcely) plausible deniability. For lower profile targets they are happy to use more direct and reliable means such as “falling” down stairwells or straightforward shooting. I don’t think that many western nations can take any moral high ground on the issue of assassinations, most have done fairly grubby things in the past.
The problem is that Putin sets the threshold for extrajudicial and (most seriously) extraterritorial killings far too low for a rules based world order. If everyone behaved as Russia currently do then it would be far more difficult to maintain global stability.