According to some defenders of abortion, what makes it wrong to kill an adult but not wrong to kill a fetus is that the adult has future-directed desires while the fetus does not. But now imagine an innocent adult who has only one future-directed desire: to die. Nonetheless, it is uncontroversially wrong to kill this adult without her consent (it's wrong to kill her with her consent, but that's controversial). Thus, it is wrong to kill this adult without her consent. Why? Well, on the theory in question, it's wrong because she has future-directed desires, or, more precisely, a desire. But the desire is a desire not to be alive. It is absurd that the presence of that desire is what makes it wrong to kill her.
So what makes it wrong to kill her? I see two answers: The first is that she is being deprived of future life. And that future life is valuable even if she does not recognize it as such. The second is that the killing is a destruction of a human body.