I agree this has a lot of potential.

Why is this note mysterious? Why has his wife been murdered?

Tormented private investigator--why tormented? What drives him or why must he solve the murder?

serial killer--how so what brings the investigator to believe this? What is the mo? Why is this person killing? Is there something in their childhood or other stages of their growing up or adulthood that caused them to kill or is it medical/genetic?

Is there something that they alone came into contact with that triggered this killing spree? Chemical/Si-Fi, inorganic, or some unknown until now thing?

What if this human had something done to them in a secluded place by someone or something? Why would this happen?

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