@JilltheTooth I’m not criticizing. It was an actual clarificatory question. Still, there’s a difference between heroism being relative and recognition of heroism being relative. No one is going to deny that who someone considers to be a hero might vary by context. Still, that does not entail that who really is a hero varies by context. Hume actually discusses this quite a bit in A Treatise of Human Nature.
One thing he considers is how we are all capable of taking on what he calls “the general point of view” wherein we neutrally recognize that people who we dislike because they are our rivals or opponents have qualities that we consider positive. That is, he notes that we can actually recognize something like heroism in people who are against us when we stop focusing on our personal conflicts.
This ability, one might argue, is needed to formulate relativism in the first place—yet it might also undermine relativism by emphasizing the distinction with which I opened this comment. I’m not necessarily endorsing either side of the argument, but I think it’s something interesting to consider.