Personally, I think assassination markets could be a net benefit (think of the bounties on someone like Kim Jong-Il), but then, I like them enough that I write stories about them so I'm biased: http://www.gwern.net/fiction/The%20Ones%20Who%20Walk%20Towar...
Did you ever think of the bounties on anyone inconvenient to governments or big businesses? The founders of The Pirate Bay, Julian Assange and Bradley Manning, BP whistleblowers, environmental activists and climate scientists – would society be better off with them all dead?
So, yes, I did think of the inconvenient people.
Now think of the bounties on average folks like you and me in comparison.
That's right, our lives will instantly be worth pennies and there'll instantly many folks around with incentive to murder.
While an expert assassin may take a job on a random Joe for on the order of a hundred dollars, an amateur with any sense will not, and therefor the price will be rather high.
And, even if assassinations of pedestrians becomes realistically common..On the order of car crash mortalities, its at least a cause of death that may make people consider how they act toward one another.