Those friends who often used Dark Magic to kill and burn in the HP world should know a basic fact.
After killing someone, cracks would form in a person's soul.
And if those friends also frequently split their souls to create Horcruxes, they would definitely be very familiar with such cracks.
After all, these cracks were an essential prerequisite for splitting the soul. With them, paired with a specific incantation, one could tear off that piece of the soul. Then, with various Dark Magic spells and a vessel to contain the soul, a fresh new Horcrux would be complete.
Of course, some friends might have wondered whether splitting one's own soul like this would cause one's magical attainments to decline. After all, the books had never recorded anything about it.
As for this question, Riddle, the renowned Hogwarts graduate and respected senior, firmly believed that true knowledge came only through practice.
To fill this gap in magical literature, he personally made six Horcruxes and, inadvertently, created a seventh. In the end, he made up for this omission in the academic history of Horcruxes—
Even if a soul was divided many times, the Wizard's own strength would not be greatly affected. Their mastery of Dark Magic might even improve further if they killed enough people and accumulated sufficient malice.
But sanity... if their souls had already been split, what sanity was there left to speak of? How were they supposed to stay rational? Mad thoughts had long since seized the high ground of their minds!
So it was perfectly normal for Voldemort to often do unreasonable things. He no longer even had a brain, so why demand high standards from him?
Even so, finding a group of allies who looked like Wizards on the surface but were not human within was