The First Luisa Rey mystery is all about conscience, and Joe Napier puts this explicitly into words, wondering if "his conscience [has] got an off switch?" (3.10.13). Well, conscience doesn't have an off switch as much as it does a dimmer switch. Napier turns it down for a bit but cranks it back up by the time the novel is over.
We learn that Napier made some bad decisions before the book starts. He was part of the raid on Margo Roker's house, the one during which Bill Smoke beat Margo senseless and put her in the hospital. But since he only had a few months before retirement, Napier put his conscience on the back burner. Then Luisa Rey came along. Not only does Napier remember her as a girl, but he knew her father. Her father saved Napier's life.
We've seen this I'll-save-your-life, you-save-mine thing before, with Adam Ewing and Autua. This one has a different spin. For starters it's I'll-save-your-live, you-save-my-daughter's this time. Plus Napier's conscience won't let him just help Luisa once and feel like things with fate are square. He has to see it through to the end—and it turns out to be his end.
Saving Luisa's life ends up exposing Seaboard's corruption. But it also leads to Napier's death in a shootout with Bill Smoke. "In the fourth to last moment of his life, Napier fires another bullet into the marionette silhouetted by stars. The word Silvaplana comes to him, unasked for" (9.68.11).
Silvaplana is the pier where Lester Rey saved Napier's life. If Napier had accidentally shot Lester that night (he came close), Luisa wouldn't be here. If she hadn't been alive, Napier wouldn't have been killed in a shootout with Smoke (assuming he had survived that night at the wharf)—but then thousands of people would probably die in a Swannekke disaster. What does Fate have in mind here?