What quite made Amy into the person who would do all this is never explicitly said. The way her parents have, through the Amazing Amy books, corrected all wrongs in her life suggest this is her taking a similar tact; whenever something doesn't go her way she adjusts her life to make it the perfect image (Tommy found her too intense, so she accuses him of raping her). The way she believes all relationships involve both partners striving towards an ideal outside of themselves (while arguably having a ring of truth) suggests a lack of real empathy; she desires the feeling of love, hence why she goes in strong with guys in an attempt to capture that, but as a sociopath is fundamentally unable to fully empathise and finds herself adjusting herself in the belief this will make things work. When this reaches an extreme, with circumstances pushing her relationships apart or the lack of feelings making them feel perfunctory, she feels wronged and then the real psycho comes out. She clearly understands emotions from the way she could convincingly write a diary implicating Nick, but whether she ever actually felt them isn't clear and there's definitely a sense of aspirations here. The ending really highlights this, with her ending one life and beginning another just because Nick going on TV and showing a knowledge of how her mind works makes her want to be with him again.
