The act of creating a horcrux is considered to be dangerously unnatural.
Tom Riddle: "I still don't quite understand how that works, though, sir."
Slughorn: "Well, you split your soul, you see, and hide part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged. But of course, existence in such a form... few would want it, Tom, very few. Death would be preferable."
-Harry Potter and the Half-Blood Prince by J.K. Rowling
Bellatrix: "No, there are pieces of you missing!"
Voldemort: "Are you talking about my horcruxes?! Because if it wasn't for them I wouldn't even BE here right now!"
-A Very Potter Musical