The psychology of killing is a whole separate can of worms from what we are talking about. Indeed, its the subject of a new Chapter in my up-coming
Book of Broken Dreams revision.
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[/shameless plug]
For now I'll refrain from an essay on the psychology of killing and say only that it is very different than what we are talking about here (for reasons cited in my post in the other thread).
There are other things that keep you from killing besides laws and morals.