I can and have acted “instinctually” many times in situations where action had priority over consideration. As a 6year old I saved my 2year old sister who had fallen into the garden pond. I could not swim either, though I knew the pond was not very deep.
Often, a person gives his life for a “cause” other than humanity. This cause may be different (contain false information) from what morally convinced the person to offer his life for humanity to begin with.
But, “Martyrdom for Humanity? Who to trust?
Someone comes to me and tells me that it is my life or the lives of all other humans on earth. Such a scenario seems a stretch to me. My answer would be no.
But spontaneous action in a critical situation, where all considerations are secondary to a confidence in oneself to be able to do it. Perhaps it is a genetic animal response which instinctually turns aggressive toward a common enemy or dangerous situation.
Ants and Bees are prime examples of genetic programming for martyrdom in hive defense.
Then also, why do mountain climbers risk life and limb to climb Mt Everest? The answer, “Cause it is there and I can conquer it”