Riddle Me This

According to a psychologist who developed this question, all serial killers to whom he posed it answered the same way.

Answer this question:
A woman goes to her mother's funeral. There she meets a man with whom she is instantly smitten. She has such strong feelings for him that she comes to realize that he is her soulmate, and they are meant to be together. They do not, however, exchange phone numbers. Afterward she finds herself thinking about him constantly. Two days later she kills her sister.

Why?