Bluntj:Someone needs to "get him". He is far worse than Saddam ever was in terms of war crimes.
That's probably not a very good idea for the same reason the Allies allegedly never tried to kill Hitler during WWII [*], his incompetence and constant meddling was one of their greatest assets. In Putin's case he's doing an effective job of holding the whole mafia state together and keeping anyone truly competent who might turn into a threat to him out of play, you don't really want to change that.
[*] There's no strong evidence that this was ever an official policy, however not encouraging any internal opposition to Hitler was since if he'd been removed it would have denied them total victory.