The "ham-fisted" assassinations have always been about just the tiniest sliver of deniability while definitely sending the message "we can reach you" and not making a secret about who "we" is.
I wouldn’t be surprised whatever perverts Putin had conjured up to make him new poisons managed to make one that manifests similar symptoms to leukemia. It’s not beyond him to try new ones out on his subjects who have fallen out of favor.
What does a corrupt mob boss with near limitless power over a nation get up to in his leisure time? I would imagine atrocious shit has gone down in that palace.