The plot

First there is Gloria's death. It looks like a simple robbery. Then, Gloria's death is linked to James Cordell's death because the killer is the same. It then appears that Glori'as death was not a robbery. Indeed the killer has kept a cross earring, which proves a certain attachment to the victim. He must have chosen his victim. McCaleb is convinced there must be a souvenir stolen from James Cordell too. Buddy is the one finding the souvenir, it is a family's photo who was taken from the victim's car.
Above all, there has to be a link between the two victims. They were not hazard, they were preys, but how were they chosen ? At that point, McCaleb throws himself into a search for details, hoping to find one key detail. 
The scene where he finally finds the key detail to the link between the victim is quite funny. He has been looking everywhere carefully and he is waiting for Graciela to tell him about what she found in Gloria's locker. He is killing the time looking at the paper around him on the board so as to avoid speaking to the man who is with him. He is thus looking at the paper without focusing on them. That is why the answer does not strike him right away. And suddenly it hits him. The link between the victims is that they both gave blood.
The following steps come easily. The victims must have had the same blood, a rare blood, that is what made them a prey for the killer. They were killed for their organs. Indeed, all victims quickly appear to be organ donors. It all starts to make sense.
As he is working on time incoherences on Gloria's case, McCaleb realizes the good samaritan must have known about the forthcoming killing when he called 911. The only way for that to happen, is that the good samaritan is the killer. Again, once that point is clear, the rest comes easily. The killer needs an organ with a specific blood types. He finds a person who fits the criteria : Kenyon. But Kenyon's murder is a failure.He used the wrong kind of bulled and kills Kenyon almost instantly. He then tries James cordell who also fits. But 911 goes to the wrong place and the victim arrives to late at hospital. Thus he decides to call 911 before he even kills his third victim, so that she actually makes it till hospital and it works.
The last part of the plan is to turn McCaleb into the investigator investigating Gloria's death. He plans things so that a friend of Graciela calls her to tell her about McCaleb's article. Gloria makes the link between McCaleb and her sister's heart and thus contacts McCaleb. McCaleb has to accept since he has Gloria's heart, he owes her that much.