Read again...."it" refers to "mercy"
I don't think she wanted to become vegetative, or severely disabled, or whatever...do you? I think she would WANT to be whole again. But I don't think that's an option.
I think that sentence was very clear, in that "it" referred to "voluntary euthenasia". If "it" is "mercy", I find that insufficient as a reason to put someone to death in her circumstances. There's no indication that she is seeking mercy in the form of being put to death. Furthermore, I shudder to think what other extensions of the concept of mercy will be used to put people to death - I can think of no unabusable definition.
The key here for me is that because her wishes are not known, putting her to death also prevents us from learning what her wishes actually are. Given that there's doubt, what's the harm in keeping her alive, if for no other reason than to determine what her true wishes are? Being wrong in believing she wants death would make those who kill her into murderers, an irreversible situation. Keeping her alive, on the other hand is not irreversable - if alive, she can be left to die; if dead, she cannot be brought back to life. Since we don't know, can a person of conscience accept the possibility that she may be killed against her will?