That's absolutely my take on it as well. Farquar's "Some of you may die, but that's a chance I'm willing to take." Anyway, the circumstances of his death preclude treating him as a martyr for this principle. He'd have to go knowingly to his death proclaiming his belief in this principle. I.e. someone corners him and says, "ok, you're going to be one of those necessary gun deaths; do you still hold to that view?" And if he proclaimed a loud and unwavering "Yes," one could treat him as a martyr for the second amendment.
I don't think its necessary that to be considered a martyr, that the person must go knowingly and/or willingly to their death
in the moment. Ultimately, the deciding factor is more about how proponents of the particular cause
feel about the person, which gives them martyr status. However, I think that what matters, is that we, as observers, know, sense, are satisfied, based on some conduct or statements that the martyr has given us, that they were
willing to die/risk death in a tangible sense, as opposed to simply being aware of the risk in abstract, hypothetical/theoretical terms. Whether William Wallace's final moments played out in the manner depicted in the iconic Mel Gibson film is irrelevant. We know that he fought multiple battles for the cause of Scottish Independence, and so, was demonstrably, conclusively proven to be willing to face death for his cause.
So to your point, I'd say MLK's iconic "
I've been to the mountaintop" speech, his final speech, delivered the day before his assassination, falls pretty squarely into this category, but even prior to that, MLK was constantly putting himself in harm's way, in situations where he faced the real possibility of death and/or serious injury for his cause(s). That is what justifies regarding him as a martyr. In the less famous, but still well known "if I had sneezed" section of his final "mountaintop" speech, he references a prior failed assassination attempt that left him hospitalized and in critical condition such that he was within a "sneeze" of death. Despite knowing that he was constantly under very real, proven risk of death, he continued his work. So whether he said the words "Yes, I'm willing to stay my course despite knowing it will cost me my life" is less relevant than the reality of his actions. He demonstrated, on an ongoing basis, that he was willing to put himself to the peril for his cause(s).
Among the iconic lines of this speech, King mentions being grateful for being allowed to live into the "second half of the 20th century" implying that he suspects that he is not long for the world and has survived only through Divine grace/providence. But the most famous part is the "mountaintop" ending line(s), where MLK more directly implies/predicts/prophesies his impending death, by invoking the the Biblical account(s) of Moses as well as Jesus final night in the Garden of Gethsemane and His words at The Last Supper:
"Well, I don't know what will happen now. We've got some difficult days ahead. But it doesn't matter with me now. Because I've been to the mountaintop. And I don't mind. Like anybody, I would like to live a long life. Longevity has its place. But I'm not concerned about that now. I just want to do God's will. And He's allowed me to go up to the mountain. And I've looked over. And I've seen the promised land. I may not get there with you. But I want you to know tonight, that we, as a people, will get to the promised land. And I'm happy, tonight. I'm not worried about anything. I'm not fearing any man. Mine eyes have seen the glory of the coming of the Lord."
So without explicitly saying "Yes, take me to the gallows for my beliefs", MLK amply demonstrated that he was aware of the risk of imminent death he was constantly under (maybe even more specifically under at that moment in time) and was nevertheless willing to stay the course. Charlie Kirk never fit into this category. I don't think Kirk ever contemplated that he might die for his cause. I'm sure he received random threats online/in the mail etc., but I doubt he took any of it particularly seriously, as anything more than the bog-standard cost of being famous/provocative. Kirk didn't give me any indication that he was willing to put himself at risk of any real threat of imminent death for his cause(s).
One of the multiple reasons I think I'm relieved that the assassination attempt(s) against Trump failed, is because it arguably established that Trump is aware of facing a real threat of death and despite this, continues working towards his cause(s), one of which being the creation of an American fascist dictatorship in his own name. I would not want to see Trump become an American martyr, because of the power it would imbue his cause(s) with.