For Castle, the fixation comes from repeated viewings ofThe Omenand his writerly tendency to overplot.
For murder victim Gabriel Shaw, it comes from a lifelong obsession with fringe dogmas and a chemical imbalance.
(I hope everyones brushed up on their religious horror film canon!)

Credit: ABC/Scott Everett White
He showed up the home of billionaire philanthropist Warren Crowne 20 years previous to execute his mission from God.
The family survived his attack, but Shaw never let his holy commission go.
A very rich one.
What more was there for Shaw to do?
Shaw was murdered, by the way, with an implement that Lanie can best describe as a talon.
And if that word doesnt get Castles reckless fancies going overtime, then the next part does.
Lanie also finds sulfur in the wound.
Before he was a patient, Shaw was a spiritual consultant for a P.I.
named Noah Cramer, whose former office Castles business now calls home.
(Also, to make this narrative work.)
So you chose fantasy over reality?, Hayley asks.
Well, duh, Castle answers.
But whatever happened to Noah Cramer?
His widow and son are called into the precinct for an interview.
Shaw stopped by their home after his escape looking for some materials from his time working together with Noah.
She got rid of everything, she says.
Shaw demolished her husbands reputation, but this isnt just a grudge.
Shaw was evil, Mrs. Cramer says.
Shes visibly not sorry that hes gone.
Not finding what he needed at the Cramer home, Shaw eventually made his way to the office.
Hayley cracks it (I need 28 minutes and a chocolate milkshake.
The star of this macabre shrine is Victor Crowne, just 9 years old when his parents passed.
It all fits, in Castles mind.
According to the Bible, Jesus began ministering at the age of 30.
The Antichrist is supposed to come into his power at the same age.
(The Antichrists true motives are supposedly masked by pure intentions.)
Victor is candid about his hatred for Shaw, whom he blames for cursing his family.
But he alibis out of Shaws murder.
He was working in his office all night and the security cameras confirm it.
Thats good enough for Beckett, but not for her husband.
(Hes not going to be bound by earthly laws, Beckett.
Thats Demon 101.)
He returns to the chamber where Hayley and Alexis are cataloging Cramers possessions.
He did anoint Shaw with the Latin phrases that were on his body when he died, though.
Beliefs aside, hes not one to turn down 50 bucks.
The team is getting closer and closer to the end of Shaws timeline.
But theres nothing otherworldly about the solid evidence that comes from a new eye witness.
Mrs. Cramer doesnt know anything about a robbery of Richard Castle Investigations the night before, however.
While Beckett slept, Castle followed his preoccupation with the cases religious significance back to his office.
Belief is the key.
And he left a bracelet behind.
Its identical to the one Oscar was wearing on his wrist when they met him.
Oscar von Eckland did inherit his fathers faith, despite what he told the cops.
And with Shaw dead, he believes himself responsible for wiping the devil off the face of the Earth.
Oscar survives, but the event is still peculiar.
He shakes Castles hand and makes him a promise.
You saved my life.
That means I owe you, Victor says.
Whatever you want, just ask.
Like or not, Castle may have just made a very powerful friend.