Shepard Book thought a lot about Simon, the brilliant young man willing to give up a future many would kill for to save his baby sister. But for many weeks when he thought of Simon his mind always went back to that first day when Kaylee lay on the deck with a bullet in her stomach and Simon had bargained with the Captain, bargained with Kaylee's life. Something about that moment had always troubled the Shepard, something beyond the obvious. It was only months later that he had realised what it was.
When he'd looked into Simon Tam's face that day there had been fear and desperation certainly but underneath it all a cold certainty. Book knew now that Simon had been perfectly prepared to stand there and watch Kaylee die. He would have regretted it but he would have done it. There were very few people in the verse with the will to do that but Simon Tam was one of them and that scared the Shepard. He knew what men with that iron will were capable of, after all he had been one of them once.