Question: To create a horcrux, a witch or wizard must first split their soul by intentionally and deliberately murdering someone without any guilt or remorse for their actions. Since Tom Riddle murdered countless people, shouldn't his soul have been split into more fragments rather than just seven?
Answer: Next to the act of murder, one also has to purposefully turn an object into a horcrux in order to make a horcrux. Your soul splitting doesn't automatically send that piece of soul into an object; your soul will be split but still connected to your body. As for when Voldemort's killing curse rebounded onto Harry, his real body was destroyed, and his fragmented soul shattered because it was frail and unstable, causing a piece to detach and lodge onto Harry.
The question wasn't about how to make a Horcrux. It was about why each murder Tom committed didn't shatter his soul more. For example, if Tom killed 11,000 people, then shouldn't his soul have shattered into 11,000 pieces?
I think your soul splits when you kill someone, but doesn't split again when murdering someone else (which part would?). Once you murder, your soul is split and will stay split until you detach a part of your soul. It's not like Tom could have saved up on fragments of soul by killing and then put pieces of his soul into objects one after the other. He had to murder and then purposefully put that split part into an object, and only then be able to split his soul again with another murder.
According to J.K. Rowling, random killing damages a wizard's soul, but does not split it. That requires a deliberate action using a specific Dark spell and storing the soul shard into a horcrux. Riddle wanted his soul divided into seven pieces - six horcruxes and one left in his body. Seven is a magical number. Riddle chose six objects for the horcruxes. The unexpected exception was when Riddle attempted to kill baby Harry. The Killing Curse rebounded off Harry and destroyed Riddle's body. By then, Riddle's soul had become so weakened and tattered that another piece was unintentionally sheared off, and Harry's scar became an accidental horcrux. Lily Potter's love had created a spell that protected baby Harry from the Killing Curse.
Answer: The other answer is spot on, but I would add that it requires casting a specific spell while simultaneously killing someone to make horcrux and split one's soul. (The movie downplayed this and the spell name is never revealed in the book.) Professor Slughorn had told the young Tom Riddle that the act involved dark magic, though he did not provide details. Riddle apparently discovered what that dark spell was to make horcrux.
raywest ★