If the prediction is tightly bounded (e.g. "Mr. X will be killed on Sept. 3, 2018") then you warn the victim.
If the prediction is loosely bounded ("Mr. X will be killed within the next year") then the odds won't be nearly as good, since many non-assassins may bet in favor. The assassin will have to put up a larger bet for a given payoff. Having done that, the assassin wins even if a different assassin does the work, so there's a public goods problem among assassins, and they all have some incentive to stay home playing Call of Duty instead.
If you approach it this way there is no reason all bets on the "bad" side have to have "bad" intentions.
https://www.climatecolab.org/contests/2015/shifting-attitude...
Making it easy to hire an assassin anonymously is why I find privacy preserving coins like zcash scary, despite valuing my privacy highly.
But I don't know if it ended up like that in the final version