As OrphanWilde said in their reply, when I say that it is bad to kill people because they have desires that killing them would thwart, what I was actually trying to do is taboo "person" and figure out what makes someone a person. And I think "an entity that has desires" is one of the best definitions of "person" that we've come up with so far. This view is shared by many philosophers, see Wikipedia's entry on "personism" for instance.

For example: This premise implies that if someone is easygoing and carefree, it is a lot less bad than if you kill your normal average person. To me, at lest, this conclusion seems rather repugnant. Do carefree people have a lesser moral standing? That is far from obvious.

I don't regard the quantity of desires someone has as being what makes it wrong to kill them. Rather it is that they have future-directed preferences at all. In other words, being able to have desires is part of (or maybe all of, I'm not sure) what makes you a "person," and killing a person is bad.

Also, if the quantity of desires was what made you morally significant you could increase your moral significance by subdividing your desires. For instance, if I was arguing with someone over the last slice of pizza, it would not be a morally valid argument to say "I want to eat the crust, cheese, and sauce, while you just want to eat the pizza. Three desires trumps one!"

So it's having desires at all that make us morally significant (ie make us persons), not how many we have.

Or what about animals? from what we can observe, animals have plenty of desires, almost as much or as much as humans. If we really were using desire as our metric of moral worth, we would have to value animal lives at a very high rate.

What I think gives humans more moral weight than animals is that we are capable of conceiving of the future and having preferences about how the future will go. Most animals do not. Killing a human is bad because of all their future-directed preferences and life-goals that will be thwarted. Most animals, by contrast, literally do not care whether they live or die (animals do behave in ways that result in their continued existence, but these activities seem motivated by a desire to gain pleasure and avoid pain, rather than prudence about the future). So killing an animal generally does not have the same level of badness as killing a human does (animals can feel pleasure and pain however, so the method of killing them had better be painless).

Of course, there might be a few species of animals that can conceive of the future and have preferences about it (the great apes, for instance). I see no difference between killing those animals and killing a human who is mildly retarded.