Brent begins to forgive himself for Lea's death because he realizes, first, that it's possible for someone else to truly forgive him; second, that he can still be a good person even though he did accidentally kill Lea; third, that humanity is like a giant interconnected unit and he really is a part of it; and fourth, that he has completed the whirligig project that honors Lea's memory.
This happens in the final chapter, as Brent is building the final whirligig and sharing it with the unnamed artist he befriends:
She looked at Brent. "I could be wildly wrong. But my sense of you is that you're a good person, not a bad one."
The words worked their way through Brent's brain. That he might in fact be like everyone else was a foreign idea, never considered. That he could have...
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