Ever since Emily had left him, Derek had been so focused on his heartache, that he had ignored other emotions that he had been feeling over the whole thing. And one of those emotions was disappointment. Not in himself but in Emily for leaving him without a word.
Then when his mother had accidentally revealed her role in their breakup. Derek had felt that disappointment grow because as far as he was concerned. It should have been obvious to Emily that he would not do such a thing. He was a man who prided himself on his word, and his loyalty.
She should have known that he would not hurt her in such a way. But now that Derek had heard the full story, he understood.
His mother had been playing three-dimensional chess while Derek and Emily had not even been able to find the board.