If she’s guilty there doesn’t need to be a reason why she has done it, like people suggesting childhood trauma or something ‘bad’ happening to her. She could just be a psychopath, it’s a psychological condition and she would just feel no empathy for anyone. I watched something years ago where a woman who was a psychopath (but didn’t murder anyone) explained that she would feel like a cat and everyone else in the world is a mouse and she could toy with them however she pleased and would feel nothing.