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It is suggested in many topics like here that the act of the murder itself splits the soul, so the Horcrux is created.
However, we know that Death-Eaters are killing for pleasure on the mass scale, but I can't remember from the book any other Horcrux mentioned than those created by Voldemort. So it seems to be, that although there are many cold-blood murders, only some special ones, those commited by Voldemort.
I don't have a quote under hand, but it was mentioned that Voldemort has killed many wizards, but created only 7 horcruxes. Additionally, the last one, Harry, was unintended.
So, as I understand:
- Not every murder, no matter how terrible, creates horcrux
- Horcrux is not always created intentionally
Under what circumstances, than, the horcrux is created?