The new Kelsier is something that this book needed. If he hadn’t been forced to go through the guilt of nearly-losing Vin (a reminder to him of how one of his jobs lost him Mare) I don’t think he would have had the solemnity and dedication to accomplish the things he does in the rest of the story.
He’ll return more to his old, joking self as the next few chapters pass. However, he’ll always remember what he nearly let happen to Vin, and it will become an important aspect of his character.
By the way, Vin’s line about “We aren’t invincible” is a very important one. In part two, I spent a lot of time showing off just how amazing Mistborn can be. I felt I needed to end the section with a colossal failure—and a near death—to show that while Mistborn are powerful, they aren’t by any means indestructible. Nothing’s more boring than heroes who can’t be defeated.
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