Argument: Life is an intrinsic good that cannot be traded in to end suffering
Patrick Lee. "Personhood, Dignity, Suicide, and Euthanasia". The National Catholic Bioethics Quarterly." Autumn 2001, Vol.1 No.3 - "On the other hand, a choice to kill a human life is incompatible with a love for that life: such a choice involves -- as proponents of euthanasia themselves often testify -- the judgment that some lives are not worth living, that some lives are mere means to some other condition, a denial of the intrinsic dignity of the person killed.
If one chooses to kill in order to end suffering, one sees (at least initially) that continuing to live does instantiate a human good, but that escaping pain also would instantiate a good.6 To act against the first reason, one must judge that the second reason (escaping pain) is preferable to it. But one can make such a judgment only on the supposition that the good offered by the second alternative (escaping from pain) is of a higher order than the good offered by the first alternative (human life). But it could be of a higher order only if human life were not a basic and intrinsic good. Thus, the choice to kill as a means toward escaping pain involves, at least implicitly, the attitude that human life is not a basic and intrinsic good.
One might object: it is a basic, intrinsic good, but it can be outweighed by the prospect of a greater good, such as relief from excruciating pain. However, how can one objectively measure the worth of a human life versus that of relief from pain? On what scale does one measure these two goods? In fact, these goods are quite heterogeneous and therefore cannot be objectively measured. In order for two things to be measured against one another they must have something in common so that one can have more of it than the other, or else one must simply have the sole valuable quality which the other lacks. But neither of these can be the case with two objects both of which are basic, intrinsic goods. So, the judgment that this human life can be suppressed for the sake of relief from pain is tacitly a judgment that this life does not have basic, intrinsic worth.
[...]A thing (as opposed to a state or property) can be valuable in one of two ways (keeping in mind the above distinctions): First, it might be valuable as a vehicle or carrier of what is per se valuable. If human beings were valuable in that way, then they would not be per se valuable, but only the states or properties that they bore or carried would be of per se value. Or, secondly, a thing might be valuable because it is per se valuable, that is, it is valuable for its own sake, and not as a means toward what it enables to be instantiated. But, human beings must be valuable in the second way rather than in the first way. For if they were valuable only as mere vehicles for what is per se valuable, then it would always be morally right to kill one child, provided one agreed to replace him with two others. No human beings would have more than replaceable value. None would have the kind of value that almost all of us recognize that at least some human beings do have. So, human beings are intrinsically valuable, that is, valuable per se. This means that they themselves are valuable, not just as vehicles for what is valuable."