Depends. Could I? Yes, I know a lot of different ways to kill a person. Could I bring myself to do it? Yes, under the right conditions. If they were endangering my life or the lives of innocent people, I'd kill them in a heartbeat if I was able to. Could I live with it? That's another thing entirely.
I treasure life above all else, and to deny someone of that life is a crime that just about beats all others, at least to me it is. I could do it, but I'd have a difficult time living with it. No matter how much I rationalized it, I think it would still bother me. Eventually, I'm sure that the nightmares would stop, and it'd sink into the recesses of my subconscious, but every so often, like most bad memories, it would pop up.
It's not even whether or not they were a good person. Everyone has people that they know, family, friends, coworkers, classmates, it doesn't matter. Everyone has dreams that they aspired to. To deny that person their life is to deny all possibilities, good and evil, that that person could have made into reality.
Sometimes, however, there is no other option. I was stabbed by a man that was intent on ending my life because he caught me with his girlfriend. Luckily, I saw him and was able to disarm him. However, in that moment, I had the option, the knowledge and the skill, to kill him as I disarmed him. The thought occured to me that I would have been well within my rights to kill this man who had tried to kill me. But killing him would have served no purpose, and a needless killing is the worst of all. I simply disarmed him and gave him arm a good twist, dislocating his shoulder. Hurt, but not permanently so, and unable to hurt me further, I walked away.
I was angry, sure. A man had just tried to kill me, after all. But I also understood why he did it. I'd been cheated on before, and I can't say that I didn't have some feelings of causing harm to the parties involved.
Part of this is my own philosophy, and some of it comes from Aikido, the martial art that I practiced. The simple fact of the matter is that I don't like people to die meaninglessly.