What could have motivated her to kill her sister

"A woman attends the funeral of her mother. She is sad and in mourning. While leaving at the end of the funeral, she meets a guy. She finds him physically attractive and fascinating to chat with. She realizes she is just completely crazy in love with him, who would have thought after just a first meeting. He leaves and she does not see him again. Two weeks later she kills her sister. What could have motivated her to kill her sister."