Mike Resnick's Oracle trilogy is about Penelope Bailey, a girl who can predict the future, and an assassin called the Iceman who is assigned to kill her. Mendoza, the Iceman, wrestles with ethical dilemmas associated with killing someone who could change the universe for the better or destroy it.
The Oracle trilogy isn't as good as the Widowmaker Trilogy or the two Santiago books but it's pretty good. Iceman's struggle is believeable and well written, as it is in the subsequent two books dealing with Penelope.
One thing I have to mention is that the other killers Mendoza mentions are all featured in other Resnick books. That's the thing that drew me into Resnick's universe in the first place. Every bit of history or figure mentioned is featured in another of his books. The killing of Conrad Bland, for instance, is the basis of an entire novel, as are all the revolutions mentioned. Resnick's universe is tightly bound together but you won't be lost if you don't know all the details of the events.