Knippenberg’s question is interesting, and it’s one that did not occur to me in the course of writing the book. It may be a subset of a question I did consider: whether accepting death can be morally justified. I make a distinction in the book’s discussion of euthanasia–not a distinction original to me, of course–between “accepting” and “willing” death. I argue that it can be justified to take actions that can be expected to cause a patient’s death (or to refrain from taking actions that can be expected to extend a patient’s life) if the purpose of taking those actions (or refraining from action) is something other than causing death. So, for example, providing a lot of painkillers to someone to ease his pain, even at the risk of causing his death, is morally distinguishable from providing the exact same amount of painkillers with the intent of causing his death. It’s the structure of the intent that matters. (Note that I’m talking about a moral distinction, not a legal one, since the two can’t track each other perfectly.)
I think the case of martyrdom would be in key respects similar. The martyr may accept and even welcome death. He may take actions that he knows will lead to his death, such as defying a tyrant. But the purpose of his defying tyranny, witnessing to his faith, or whatever other action leads to his death sentence, is not to be killed. He feels morally obligated to do those things for non-death-wish reasons. If he could defy the tyrant and otherwise act uprightly while avoiding death, he would. His death is thus an unintended side-effect of his acts. The martyr’s relatives may tell him that he is committing suicide, but we don’t have to see it that way.