First you have to conclude that the person who was performing the heroic act actually thought the outcome would be death...unless they are jumping in front of an active shooter, which few do, most performing heroic acts more than likely think they can accomplish successfully whatever endeavor they are undertaking. For the rest are they acting out of throwing their life away or out of a natural instinct to protect others, if it's instinctive it's a reaction without forethought negating thoughts of throwing one's life away.