Interesting story.
However, very many ethical questions for the PI.
He did the right thing, persuaded Nicholas Holbert, the killer with anger issues, to confess. Nicholas Holbert was probably a walking time bomb anyhow, very dangerous, so good job, PI!
And now comes the question of ethics. Nicholas was an odd, probably learning disabled, extremely isolated man, without ever having a room, a corner in the house, to live in. Living in the woods, like an animal. A jailbird, with the history of adversity, without any family or stability in life.
And to this man, the PI offers something that Nicholas, probably, never had - human warmth and friendship. He is the friend in need who would borrow you the money. So he tames the loner, gets his story, pays for a motel room, and in the morning is surprised that NH has not absconded. (How would he, he lived in a tent under the rain, and here his friend is paying for a warm motel room).
Afterwards, everything goes as scripted (and what a miserable person, NH! Probably had traumatic encephalopathy, or something similar). And the court, where he tries to say, I am sorry.
Everything ends as it should have, the killer is behind bars. I have only one question.
When the PI received the letter from prison, from NH, he did not answer.
I would have. NH was a total loser who never had anything good in his life and ended up killing a vibrant young woman.
But the PI... as Antoine de Saint-Exupéry has said in his immortal “le Petit Prince”, “you become forever responsible for what you’ve tamed”.
You tamed him - you answer his letters now. You can’t push him away. The man is in prison for life. And if you have solved the case by befriending the loner, maybe, here is your own chance at compassion that we always try to teach others.
Now tell me how I am wrong.