a few thoughts about the end

The key to finding the killer was in the hypnothis session. As he watches the tape again, McCaleb realizes Noone did not answer one question though he should have answered it. If he was really hypnotized he could not have known the question was not adressed to him but to Winston. The fact that he did not answer proves he was faking the whole thing. As a consequence, he must be the killer !
 
In the end, McCaleb kills Noone, whose name is actually Crimmins. The gesture might surprise the reader. Why does he need to kill him, why doesn't he just send him in jail ?
Crimmins used to be the one chased by McCaleb. He used to kill people. He was evil. McCaleb was just the good man trying to solve the crimes. He has managed to reverse the situation. For a while, McCaleb is the one being chased by Crimmins. If he had kept to his role of investigator, he would have left Crimmins to the police. Crimmins would have gone straight to jail and that was enough for him, Graciela and Raymond to be protected. True, it would have implied more trouble for them too. A part of his choice to kill Crimmins might have been to spare Graciela and Raymond the trouble of what was going to happen next. Perhaps the last trick of Crimmins was too much for him. He kidnapped Graciela and Raymond, threatening the life of two people who at the end of the story mean the world to McCaleb. Whatever his reasons were, McCaleb acted as a man involved in an awful story and not as an investigator solving a case. He was 100% personnally involved in the case, contrary to all his previous cases. That is why he did not react the same way as before. His gesture was wrong but understandable.

I don't think he was weak and failed to separate his private life from his work. From the very start, he is personnally and emotionnaly involved in the investigation. He does it as a favour to a woman whose sister's heart is now his. He recognises he owes her that much. That is very different from investigating a stranger's death, however awful the death was. There was more to it than just compassion and horror when facing evil.

Even though he lies to himself at the beginning and tries to play things as if he were just an investigator without a license. Little by little, reality catches him. So here is how I understand the end : McCaleb has finally managed to accept himself. At the very moment he choses to kill Crimmins, he is not the investigator, but a man. A man who owes his life to Gloria, a man who has been psychologically tortured by Crimmin, a man who is in love with Graciela.