What Mental Disorder(s) is this?

I'm not familiar with a specific disorder that you're describing. However, there are many serial killers who suffer from Antisocial Personality Disorder which prevents them from feeling sympathy or empathy towards another person. They cannot identify with the pain they are inflicting.

On the other hand, someone who kills in order to calm themselves points to an anxiety disorder, which Antisocial Personality Disorder is not.

Maybe it would help to poke around on this site: www.nimh.nih.gov (nimh stands for National Institute of Mental Health). It's a good reference for educating yourself on mental disorders. Also if you do some poking around on the internet and come across and references to the DSM IV that should help as well (DSM stands for Diagnostic and Statistical Manual 'of Mental Disorders'). It is the Psychology Bible, though after 4 revisions is far from ideal.

I think this person is what is commonly referred to as a psychopath/sociopath. I think they would probably get diagnosed with Antisocial Personality Disorder, but there are many others that could fit in their as well. Interesting question!

Narcissistic Anti Social Personality, amongst MANY other things.

It might be because they are afraid of someone, but once they have killed what they feared then they can relax and calm down until they are afraid again.

idk O_O