At one point, Harry asks:

“Sir … is it important to know all this about Voldemort’s past?”

“Very important, I think,” said Dumbledore.

This isn’t quite the same as the criminal profiling that the FBI and forensic psychologists do, because of the critical difference that we already know who the perp is (Pale, tall, snake slits where his nose should be). In criminal investigations, the point is to take the crime and figure out what the psychology might have been behind it, in the hopes of catching the criminal. The “why” is employed in service of the “who.”

Of course, when the killer you’re after is the most powerful Dark wizard of all time, simply knowing who he is isn’t enough to catch him. And catching him isn’t enough to stop him, since he inconveniently split his soul into seven pieces and hid six of them in objects called Horcruxes. To kill him, all seven pieces must be destroyed. Harry and Dumbledore are wading through memories to confirm Dumbledore’s suspicions that Voldemort created Horcruxes, and to figure out which objects he might have chosen. The “who” is employed in the service of the “what” and the “why.”

As in real criminal profiling, this endeavor involves a lot of inferences and conjecture. “From this point forth, we shall be leaving the firm foundation of fact and journeying together through the murky marshes of memory into thickets of the wildest guesswork,” Dumbledore says.

What they discover about young Tom Riddle conforms to many of the classic stereotypes about serial killers—his parents were unhappy, he was a loner, he bullied other children and tortured animals.

Most importantly: “The young Tom Riddle liked to collect trophies,” Dumbledore says. “You saw the box of stolen articles he had hidden in his room. These were taken from victims of his bullying behavior, souvenirs, if you will, of particularly unpleasant bits of magic. Bear in mind this magpie-like tendency, for this, particularly, will be important later.”

The collection of trophies from victims is another serial killer trope, and it comes back into play with the Horcruxes—Harry points out that Voldemort could store his soul in any old thing, making the Horcruxes impossible to find.

“But would Lord Voldemort use tin cans or old potion bottles to guard his own precious soul?” Dumbledore replies. “You are forgetting what I have showed you. Lord Voldemort liked to collect trophies and he preferred objects with a powerful magical history. His pride, his belief in his own superiority, his determination to carve for himself a startling place in magical history; these things suggest to me that Voldemort would have chosen his Horcruxes with some care, favoring objects worthy of the honor.”

Dumbledore proves to be right about that.

In this storyline, the sixth Harry Potter book in part resembles a true-crime book, and is intriguing for a lot of the same reasons. Stories of real-life serial killers captivate less because of the crimes they commit and more because of people’s desire to understand why someone would do those things. As I previously wrote in a story about serial-killer celebrity:

As the retired NYPD homicide detective Dave Carbone [said] when asked about the public’s interest in serial killers, “The why is the wow.” Or in the words of Katherine Ramsland, a forensic psychologist … “It’s not really about the victims. It’s more about the puzzle—the interesting labyrinth of human emotions and human motives.”

Voldemort is motivated by immortality, superiority, racial cleansing, and more than a bit of self-hatred. Many of these things are shown in earlier books in the series, but become crystallized in the sixth. Tom Riddle has a witch mother and a Muggle father, making him a half-blood wizard. But as Lord Voldemort, his ideology is centered around the superiority of “pure-blood” wizards, and his desire to rid the world of Muggle-born wizards and half-bloods like himself. Rowling has acknowledged that Voldemort is similar to Adolf Hitler in this way.