In an earlier post this week, I argued that if one accepts the Interest Thesis (IT) that what makes a killing of an innocent person wrong is that it goes against the victim's interests, one needs to hold that it is never in the interests of a person to be killed. Hence, if IT holds, the justification of euthanasia in terms of the interests of the patient/victim fails. I ended the post by mentioning an account on which a person's consent is what makes it acceptable to kill the person, without settling the question whether that would be a good account. I will now try to settle this in the negative.
Why would x's consent make it permissible to kill x? Consent (and I use the term to mean "valid consent", consent satisfying whatever kinds of freedom and knowledge conditions are needed) is tied to autonomy. Generally speaking, when consent makes it permissible to do something which is impermissible without the consent, that is because doing the action without the consent is a violation of the person's integrity. Is killing a person without her consent a violation of the person's integrity? That claim seems to have a lot of plausibility.
But why does non-consensual killing violate a person's integrity? Here, we have to be careful. It won't do to say that such killing makes it impossible for the person to fulfill her life's autonomous projects. For it might be that the person's life is in such a miserable state that she has no projects that are interrupted by the death. She may want death, but still not consent to death. It need not even be an autonomous project of hers to continue living, just a morally driven determination not to consent to death.
Perhaps we want to say that non-consensual killing violates a person's autonomy because death is always a very serious harm and we should not intentionally impose very serious harms on innocent people without their permission. But I think we can simplify this principle to just say: "We should not intentionally impose very serious harms on innocent people", and then all intentional killing of the innocent is forbidden. Moreover, even if we don't go for this simplification, if we think death is a very serious harm, then we are apt to think that someone requesting death is irrational or constrained by circumstances, both of which endanger the validity of the consent. At this point it will be really implausible that it is the lack of consent that makes murder be wrong—for the serious badness of death is entering into the story.
Or maybe, in line with the element of formalism in Kantian ethics, we will say that the reason it is wrong to kill someone without her consent is that it is wrong to significantly change a person's life without her consent.
But this change principle is inapplicable and/or false. First of all, it's not clear that terminating a life counts as "changing a life". Moreover, if it counts, then so does beginning a life. But a couple does not need consent from their future child to conceive that child. Second, there are many significant life changes that it is permissible to impose on a person. It would not be impermissible for me to offer you a billion dollars, if I had it and if I had no morally obligatory alternatives for my largesse. But the offer would significantly change your life, and it would do so without your consent. For either you would accept the offer, in which case your life would significantly change (assuming you're not already very rich), or you would reject the offer, in which case your life would also significantly change, for instance because for the rest of your life you would from time to time be reflecting on your decision and wondering if it was the best one. Nor would I need to ask a person her permission before offering her a life-changing argument. Granted, she would have to consent to the change of life, but if the argument were powerful, her life would be forced to change: either she would have to act in accordance with the argument or else she would have to become the sort of person to whom arguments do not matter. Examples like this can be multiplied.