How to mangakas come up with such plots?

I was wondering how do they come up with suck disturbing plots i read some physiological mangas and omg it was hard to read. Not that it was bad but it was really good that it was painful to read how can people come up with this stuff. After im done reading one i become really disturbed like how can such a thing happen im shocked. For example im reading black butler and it's kinda depressing some mangas get too dark. Maybe it's me since i can't handle such hard stuff but still don't the authors get affected by it. Someone explain.