"A" is for Alibi

I was surprised to read first sentence of this story. The first sentence was "The day before yesterday, I killed someone.". I thought "Are you OK?", "Are you a detective?", "Why can she continue a job of detective who killed other people?". I had a lot of questions for her and her story.
I read some chapters, there were many characters. I'm not good to remember each person characteristics, so it makes me to confuse this story. I think she was a good detective.

Question 11-1. Gwen tells Kinsey that Colin last saw her at Diane's junior high school Graduation. Kinsey doesn't believe her Why not?

Answer: Colin was about fourteen months old at the time of that graduation. If he last saw Gwen then, he wouldn't remember he at all.