Briony’s self-delusion is so complete that even the twins’ safe return, an unequivocally good thing, causes her outrage because it mars the perfect story she has devised. She has made herself believe her narrative, or perhaps it is more accurate to say that her narrative has overcome her, so that she can only see Robbie doing a good thing as him hiding what Briony is certain is his true depraved self. The speed with which Robbie is determined to be the sole suspect likely has to do with his lower social class and the biases held against him for it. He felt a part of the Tallis family, but he and his mother were always actually outsiders granted contingent access.