Do you honestly think that it's selfless to write a happy ending? SELFLESS? As in, Briony is only thinking of others, and not herself, when she wrote that ending? The difference between Briony's ending and the real ending is HUGE: THE TRUTH. The truth is the difference between Briony's happy ending and the tragic real one, and to sugarcoat is being the God of her own universe. It wasn't enough that she had to engineer their lives while they were living: she has to pay them the ultimate disrespect by continuing to fuck with their lives after they'd died. It isn't a kindness, it's desecration. Briony did not write this book to tell a nice story to an audience, she wrote it because she had to, and she did it to allievate her guilt. She may know that she is accountable for her crime, but she's still trying to find a way to atone, and through writing this book- HENCE THE TITLE- that is what she is achieving. Atonement doesn't mean forgiveness, it means reparation for a wrong, it means to make amends. Briony wrote the story in an attempt to atone for her sins, and she can't do that by way of the wronged, because they're dead, so she's trying to get it anyway she can.

Briony wrote a happy ending, but she still didn't let herself off the hook entirely

Entirely. Didn't let herself off the hook entirely, but she wriggled off a it, and relieved herself. Writing that book with that ending was wriggling off the hook to make herself feel better. There is nothing but selfishness in that act.