I understand the impulse to kill, I do. To my shame, perhaps, because I have in the past said "I wish s/h/it would just die" and meant it. Not that I think about killing people all the time, but I have encountered people in my life who I honestly thought did not deserve to be alive because they were awful people, because they did nothing but hurt others, because they seemed a waste of space and a waste of humanity. But you know. Social boundaries, the voice of conscience, good old-fashioned fear of consequences -- all these things, coalescing in my brain and making me different from a crazed Bonobo chimp even though 98% of the chimp's genes are identical to mine. Even if I was faced with a heartless person who bludgeoned kittens for fun, who had no family, no friends, no job, no future, I don't think I would be able to kill them, even if I did not need to fear the consequences of my actions. I think it is unjust to take things that don't belong to you away from people, and the only life that belongs to me is my own.
I am no humanist. Though I have great respect for humankind as a whole, as a formless concept, an idea of something "greater", a collective of souls, of intelligent beings, I do not feel that every human being is special and unique and wonderful and must be saved/helped/elevated, no matter how atrocious their behaviour. I believe that some people are rotten, and I have no sympathy for them when they die or when they suffer, but I could not kill them myself.
I understand hatred. I understand being so hurt that it seems that nothing will help but hurting the one who hurt you first. I understand wanting to destroy someone who threatens your life or that of someone you love. I understand those things because I have at one point or another experienced them and only a vague, almost formless thread separated me from acting on what I was feeling. I suppose I usually call that thread "sanity", because even social norms, fear of consequences, and my conscience can be drowned out by primal fears that have nothing to do with the world and everything to do with a living organism's striving to survive the best way it knows how.
I understand war, and battle rage; we are nature's crowning achievement, after all, and we are just as warlike as a pack of lions on the hunt, except we have the aforementioned social norms, the voice of our conscience, and fear of consequences tempering our impulses; products of a civilisation that would not exist if not for society-building. I understand a jealous wife murdering her husband. Sex and reproduction play an enormous role in our psychological make-up, and the feelings unleashed as a result can destroy even the staunchest of virtues. I understand a mother killing her child because she is convinced the child was sent by the devil. I understand clinical insanity; I studied it with great interest for a number of years before I realised that I could not possibly spend my life trying to fix people who don't want to be fixed because they don't feel there's anything wrong with them.
I can wrap my mind around a lot of morbid, horrible things. I don't like to, truth be told, but I can. However, for all that, I do not understand why anyone would walk into a school, a library, a university's commons, hell, any public place, and open fire on innocent people the shooter doesn't even know. I just don't. I know there's nothing logical about it, but even illogically, I do not understand why anyone would ever do that. And yet it keeps happening, and no one seems to understand why. It's days like this that I am forced to remember that our world is a seriously fucked-up place.