In Whirligig, why does Brent begin to forgive himself for Lea's death?
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 done what he'd done and still be good was an even more startling notion. He remembered the note from the motel maid: "No one is alone with Jesus." Jesus forgave you no matter what you'd done. But that was his business, and the priests' and ministers'. They were professional forgivers. They said "It's okay" the same way your parents said they loved you, whether they meant it or not. This, though, was different--hearing himself forgiven freely, by someone he trusted. He wasn't sure, though, that she knew enough to forgive him. He told her the story in detail. It didn't seem to change her mind.
Soon after talking with the artist, Brent heads back to his temporary lodgings, and stops along the way to join a group of dancers. This is when he realizes that people as a group form meaningful, interconnected units, and that he can still be accepted--he can still become part of a group of people and belong with them:
It was exalting to be part of the twining and twirling, and strangely thrilling to touch other hands and to feel them grasping his. He felt like a bee returning to a hive, greeted and accepted by all. [...] It felt to Brent like his rite of reentry.
All these realizations lead Brent to begin to forgive himself for Lea's death. Having completed the entire whirligig project also helps, too, of course: it's what Lea's mother wanted him to do, and by fulfilling his promise to her, Brent feels like has made restitution for his crime. Now, even though the guilt will never truly leave him, Brent feels ready to go back to his family, to his school, and to his regular life.