Would you kill somebody? For real? With a lot of blood and suffering? Slowly? This person could be begging for his life and you would not give a shit?
Not I, and certainly not without reason. I suppose the only justifiable reason is for defence and possible revenge of a loved one, and even in that case I would still give a shit.
I have a fascination with serial killers, but it is based upon my inability to relate to their reason for committing the crimes they have.