Does accidental homicide cause one’s soul to split?
We know that in order to create a Horcrux, murder is necessary:
Riddle's hunger was now apparent; his expression was greedy, he could no longer hide his longing.
"How do you split your soul?"
"Well," said Slughorn uncomfortably, "you must understand that the soul is supposed to remain intact and whole. Splitting it is an act of violation, it is against nature."
"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."
Harry Potter and the Half Blood Prince Chapter 23: Horcruxes
Once someone commits murder, they can then use the fragmentation of their soul to create a Horcrux. But my question is, what qualifies as murder? Normally I would assume that murder is plainly using Avada Kedavra on someone, or some equally lethal spell, or by one's own hand causing them mortal injury.
But in this answer, J. K. Rowling is quoted to have said that the diary was made into a Horcrux when Moaning Myrtle died. However, she was not murdered by a spell, nor with an inanimate object, but indirectly through a Basilisk. Quite likely the Basilisk would have killed Myrtle whether or not Riddle told it to. Still, this counted as murder.
What, then, counts as a murder that splits one's soul? If someone orders a hitman to kill somebody else, does that count as murder for the first person? If someone invents a weapon intended to kill people, and somebody else uses it for murder, is the inventor's soul affected?