Despite the complex and finely-woven world creation at play in the Harry Potter universe, very little is known about one major process:
How is a Horcrux created?
Reddit user sirlionel13 posited an interesting theory: Voldemort was a cannibal.
What we know about creating a Horcrux
Professor Slughorn tells a young Tom Riddle that killing a person is vital for the making of a Horcrux:
"An act of evil — the supreme act of evil. By committing murder. Killing rips the soul apart."
However, murder alone is not enough
Dark Wizards and Aurors go round killing people all the time and the world isn't swimming with Horcruxes - in fact, we're told they're rather rare. Clearly, simply killing a person is not enough to create that destructive soul bonding and tearing needed to create a Horcrux.
What's the Quickest, Darkest Way to Consume Another's Power?
According to the mythology of many different cultures, the act of ingesting another person's flesh was a way to absorb their power. In fact, the Smithsonian reports that even in 17th century Europe, consumption of cannibalistic medicines was rife:
"Human remains were considered potent was because they were thought to contain the spirit of the body from which they were taken. “Spirit” was considered a very real part of physiology, linking the body and the soul. In this context, blood was especially powerful. They thought the blood carried the soul."
It's certainly an interesting theory
The 'Cannibal Horcrux' theory is satisfying in that it ties into ancient ideas of magic and taboo - Voldemort's killed a baby and a unicorn, committed a genocide - why wouldn't he be OK with cannibalism? We're told he ate other snakes during his time as a snake - maybe his feelings of affinity with these creatures and/or general lack of empathy mean that cannibalism as a human wouldn't put him off...
The true Horcrux process remains a mystery
The true secret of the Horcrux is hidden in the book Secrets of the Darkest Art. The book was housed in the restricted section of the Hogwarts library, then in Dumbledore's personal office, before it was borrowed indefinitely by Hermione to destroy Voldemort's Horcruxes.
I guess we'll just have to enjoy speculating until J.K. Rowling throws us a bone... a human bone.