The technology to do it now fully exists. Zero Knowledge Proofs. The potential for a someone with a lot of money to put out bounties on whoever they want to, now exists. A fully competitive market in bounties could be implemented.
After all, it's not like assassination isn't a business of politicians. A lot of critics of a certain black widow power figure seem to be suiciding. And assassinations don't have to be snipers in the upper story... a seeming mosquito bite, a heart attack...
I'm not advocating it but I think that sooner or later, now that everything this hypothesis depends upon exists, we could see this actually happen. There was in fact a few hitmen, or at least scuttlebutt to this effect, on some of the black market sites some years ago.
I mean, it even doesn't have to be literal assassination, it could be bounties for someone to get credible information leaked that destroys someone's political career.
the young man suspected of leaking the DNC crap to Wikileaks was shot dead in broad daylight.
Hitman?
No, most likely FBI.
As the reality of an anonymous, decentralized assassination market sank in, nation-states wouldn't collapse so much as surrender, devolving into smaller and smaller states until the free market supplied all necessary services.
For a brilliant analysis of, among other things, the role that insurance companies would play in this regard, see Hans-Hermann Hoppe's "Democracy: The God That Failed": http://www.riosmauricio.com/wp-content/uploads/2013/04/Hoppe_Democracy_The_God_That_Failed.pdf