I can't rule out a concerted and planned effort, but there's no need to go there. Russia has a lot of people trained in the use of poisons, and a lot of them have (or can be hired by people who have) people they'd like to be rid of. Without any coordination, without any central planning, they could be acting independently while the proverbial iron is obviously hot. I don't think there's a widespread opportunity, just a lot of opportunists using the Putin fog to cover their escapes.
If I had someone I wanted to kill, and I happened to be around during the Mob wars in '30s Chicago, or the Weathermen terror campaign in the '60s, gang wars in '80s LA, etc. all I have to do is make my hit look like one of those, and then walk away undetected. I suspect that a lot of Russian scores are being settled among Russians right now because suspicion of Putin has provided a convenient umbrella.
Not that there's anything wrong with that! ;-)
Oh, Putin has plenty of his own sins to answer for, and my concern about his being blamed for the sins of others isn't because of concern for Putin; it's because I worry that other folks, just as willing to kill, will walk away lily-white and untarnished, only to pop up later.
Good points, you're could well be very right! Whatever the plot, it's more likely to be a lot more byzantine than first glance from any angle. This is Russia, after all.