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.