There are people I could cheerfully kill. I've known a number of individuals the earth would be better off without, whose mere existence I have found..... "deeply and personally insulting," is the best way I can think to say it.
I'm also not big on the sanctity of life, or the notion that everyone has a destiny to fulfill. Most humans, myself included, do not and will not contribute in a meaningful way to human culture. If I died tomorrow, the world would keep right on turning. My junk would be hauled away, and someone else would fill my house with his own junk. Others would step in to take my shifts at work, etc etc. Sure, my family would mourn, but the net effect on society would be zero. Take one drop from a bucket, what happens is nothing. It's like that for most of us. Most people, we can simply do without.
Whether I could be driven to take a life isn't the question at all. There are two questions:
1) Is it worth going to the trouble? The amount of preparation and planning it takes to defeat modern forensics is considerable.
2) How certain can I be of getting away with it? Even going to that trouble, there are always wild cards. Neighbors or family members can hear the noise, there's the slight chance a cop car could be cruising down the street at that moment, you can never clean up ALL carpet fibers/skin cells/whatever, etc. There are too many things that can't be controlled.
So far, the answers to both questions have been "not really."
That's my answer for premeditated murder.
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Whether I would kill "in the moment" is a separate question. The answer is in a do-or-die situation, e.g. shooting someone who was about to stab me, I would be indifferent to whether the other guy lived or died, I would just do what I needed to do. I would not go out of my way to kill, but I would aim for the chest rather than the less lethal (and far more difficult to hit) kneecap. By attacking me with a deadly weapon he forfeits any consideration on my part.
Make any counterargument you like about what is or is not permitted within the law. If a guy is running at me with a knife, do you think the force of the law is going to stop him? No, it is not. The force of a bullet, however, will.