What other conditions must murder fulfill to create a horcrux? [duplicate]

Question

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:

1. Not every murder, no matter how terrible, creates horcrux
2. Horcrux is not always created intentionally

Under what circumstances, than, the horcrux is created?

Answer 1

Yes, a terrible murder is not enough, a Horcrux has to be created deliberately with a spell. This much we learn from Professor Slughorn's memory in Harry Potter and the Half-Blood Prince chapter 23:

‘But how do you do it?’

‘By an act of evil – the supreme act of evil. By committing murder. Killing rips the soul apart. The wizard intent upon creating a Horcrux would use the damage to his advantage: he would encase the torn portion –’

‘Encase? But how –?’

‘There is a spell, do not ask me, I don't know!’ said Slughorn, shaking his head like an old elephant bothered by mosquitoes. ‘Do I look like I have tried it – do I look like a killer?’

Dumbledore's opinion from Deathly Hallows chapter 35 is that the unintentional Horcrux of Lord Voldemort is a special case that could occur only because his soul was already too fractured from the previous five Horcruxes he's made.

‘You were the seventh Horcrux, Harry, the Horcrux he never meant to make. He had rendered his soul so unstable that it broke apart when he committed those acts of unspeakable evil, the murder of your parents, the attempted killing of a child. […]’

[…]

‘What you must understand, Harry, is that you and Lord Voldemort have journeyed together into realms of magic hitherto unknown and untested. […]’

(See also this quote from an interview to confirm the same thing.)

No wizard before Lord Voldemort has ever tried to make more than one Horcrux, so you can be sure that this accident will not be repeated by any other Death Eater.

Answer 2

JKR specifically spoke to this issue in an interview in 2007;

I've edited for clarity and brevity:

MA: What is the process? Do you-- Is there a spell? Is there a-- What do you have to do?

JKR: I see it as a series of things you would have to do. So you would have to perform a spell. But you would also-- I don't even know if I want to say it out loud, I know that sounds funny. But I did really think it through. There are two things that I think are too horrible, actually, to go into detail about. One of them is how Pettigrew brought Voldemort back into a rudimentary body. 'Cause I told my editor what I thought happened there, and she looked as though she was gonna vomit. And then-- and the other thing is, how you make a Horcrux. And I don't even like-- I don't know. Will it be in the Encyclopedia? I don't know if I can bring myself to, ummm... I don't know.

Bluntly, you need to do two separate things to create a Horcrux;

• Commit a deliberate and cold-blooded murder in order to intentionally fragment your soul.
• Cast a Horcrux-Making Spell to remove the soul fragment and place it into an object