Well, the thing about defining a person as a human being is that I don't really get why it would be wrong to kill a human being. Human beings are made up of more or less the same stuff cows are made up of. We have similar organs and our cells look more or less that same under a microscope. It seems rather arbitrary saying that it is wrong to kill this one type of mammal but fine to turn another type into hamburger. We don't really have to worry about this today, but complications would arise if we ever found intelligent life elsewhere in the universe. Supposing they existed, would Vulcans be considered people if they are not human?
Now, I don't want to die. I think I would be rather upset if someone was killing me. I think a lot of people would feel the same way. I also am a little terrified about the prospect of simply not existing anymore. I like existing and I think it is only fair that I respect other people's right to exist. (The whole "Golden Rule" thing.) Even if I am under a lot of pain and may say things like "I wish I was dead!", I don't think I really want to stop existing. I just want to stop experiencing that pain so badly that I'm willing to risk the uncertainty of death. So I tend to define people as beings that recognize that they exist and in some manner wish to continue existing. I personally find that to be a much more convincing litmus test for personhood than making sure they have the right number of chromosomes and that the nucleotides they are composed of are in the right order.
This definition of a person has some weird consequences though. Chimps, for example, recognize that they exist and seem to like existing, so I define them as people too. Also, if someone programmed a robot that was self-aware (Sky-net

) and could profess its love for cleaning your house and computing billions of decimal places for pi, I would define them as a person too. So, I suppose killing a self-aware robot would be wrong as well, although it might be hard defining "life" and "death" for a robot so a lot of complications arise with that example. However, for now, nearly all humans and nearly all chimps are defined as people using my definition.
Also, people tend to stop existing when they faint or go to sleep. (Except when they dream.) So, you might think it would be okay to kill people when they are asleep because they do not recognize that they exist during this time and therefore cannot wish to continue existing. However, I usually get around this by saying that the person right before he or she goes to sleep expects to wake up in 8 or so hours. Damaging his or her body enough to prevent them from existing again is still denying his or her wish to exist and is effectively the same as killing him or her when the person is conscious.
However, the reason why I say nearly all humans are people is because some human beings don't recognize that they do exist. Before a baby is about 6 months old, it does not recognize that it exists. Babies of this age have no concept of "I" in the same way we do. Therefore, I generally do not classify human beings that are less than about 6 months of age as people.
Yet I still feel that it is wrong to kill humans that are less than six months old, though. I have a niece that is only about five months old and I still think it would be wrong if someone were to kill her. I also think it would be wrong if someone killed somebody's dog but for some reason I'm not as convinced that killing a cow to make a delicious hamburger is wrong. Babies under 6 months old lie in this gray area where I'm really uncertain about what is wrong and what is right-ish. There's the whole matter of my niece growing old enough to become a person as I define them and whether that should be a factor in the wrongness of killing something. Do beings that could eventually turn into people have special rights? How probable must it be that the being will become a person before "killing" it is wrong? Would preventing the programmer that is creating a self-aware robot be as wrong as killing a fetus? (Supposing the roboticist is as proficient as creating robot people as women are at creating fleshy people.) These are the bizarre thought experiments that get me hung up on abortion issues.
Anyway, sorry about the wall of text. This would be that "can of worms" I talked about in my last response.