I found this a pretty pleasant read, I didn't find it a chore to get to the end. I did find it had a lack of suspense.

Spoilers from here: Rather early on it's apparent that the narrator is going to be a murderer, so that doesn't come as a shock, and I think the author made the right decision to reveal that early on. The problem is, there is a bit of a lack of suspense as to who 'Charlie' is, the person that is seemingly following the narrator's rules for murders. For one thing, there are only really two, perhaps three possibilities, it could be the store's co-owner, his wife, the ex-cop or maybe Gwen Mulvey the FBI investigator. Again though there is a lack of suspense since it doesn't appear the narrator, or anyone he cares about is in particular danger. The murderer is copying his rules but not threatening him. The narrator is very briefly considered a possible suspect but has relatively strong alibis. What could have made this better was some chance that during the investigation he is found to be be guilty of the one murder he has committed. But that just isn't really on the cards at any point.

Finally there is the revelation at the end that the narrator actually killed his wife, but this just doesn't seem that revelatory. We already knew he was a murderer. With a final revelation that this note is actually a suicide note, but why is he killing himself? The two murders of which he is guilty (there is a third which he remembers but isn't sure is not a dream???) happened 5 years ago, so it's not any kind of remorse or feeling of guilt, he's only killing himself to avoid going to prison as when the police find out Marty has been the killer they will presumably connect the dots to his involvement in the Norman Chaney killing, though probably not his wife's. So he's killing himself to avoid going to prison, as a revelation, that is a pretty big meh for me.

Overall, not too badly written, but a pretty lacking plot makes this 'mystery' a bit run-of-the-mill.