I am conflicted about this one. I just read it again - in whole, so It is good enough, surely?
Niven writes well, no question. I am a fan of his writing, and have read many of his books.
However this one, at the end of the day, annoys me. Unfortunately, to explain why, I must refer toi specific plot points, so
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The books contains two sub-stories, and I find them both problematic, in similar ways.
In the first the protagonist finds he has to kill the villain. Moreover he is very clear that giving the villain any warning, however short, is very dangerous.
He plans an ambush where he waits, well-armed, and hidden behind cover in a small corridor where Mr. Villain is due to pass.
But, under these conditions, he doesn't just shoot the villain from cover and be done with it - no, he plans a setup where people have to jump the villain physically, immobilize him, to transport him to another room and THEN kill him. No real reason is given for this decision.
Needless to say, it goes wrong, Villain does get enough of a warning to do bad stuff before good prevails over evil. As a reader, I am left wondering why? Why would an intelligent protagonist do this?
In the second sub-story the villain is planning Armageddon. At a certain point, the villain captures the protagonist, his lover and a his friend. All three are have bullet-proof skin (trust me here - you'll have to read it to see how this comes about), but the Villain kills the protagonists friends by shooting them in the mouth. HE does this not because he finds they have bullet-proof skin. He just does this. I ask you - how plausible is this? (not the bullet-proof skin - that is part of the suspension-of-disbelief, and organic to the plot. The shoot-captives-in-the-mouth to kill them thing - when have you read this/seen it in a movie? Why would the villain do this?)
to top it off, when Mr. Villain decides to kill our hero, he does NOT shoot him in the mouth, he just shoots him in his bullet-proof torso, hence the story can go on. So, our Super-villain is suddenly inconsistent.
As a final annoyance, when he kills the protagonist's friend, the protagonist is mad "My hear died, and a man with a dead heart is a dangerous man". Then, the villain proceeds to kill his love-interest. The protagonist tells us he has now lost his heart completely, and that "a man without heart is a very dangerous enemy". So, when our hero will catch up to the villain we expect something spectacular by way of retribution. A little while later our hero manages to catch up to the Villain and - wait for it - Punches him in the nose, Once.
Seriously? This is the "very dangerous enemy"?
Not emotionally satisfying.