Are You a Serial Killer?

Husband told me that he read that nearly all serial killers have the same answer to the following question:

A woman is at her mother's funeral when she notices an attractive man who is attending. She would like to go and talk to him, but first she must deal with the well-wishers and other people who went to the funeral and want to speak with her on their way out. By the time she is finished with the niceties, the man is gone. She never gets a chance to find out who he is. A week later, she kills her sister. Why?

Posit your theory in the comments* and later I'll reveal what the typical explanation given by a serial killer is.

*Unless you know the answer 'cause you heard this before. Don't be a wet blanket when it comes to demented fun, please!