In principle, I would say that for a protagonist’s death to be ‘satisfying’ and justifiable to an audience, two conditions need to be met:

1) The protagonist has already reached a stage of personal or psychological maturity where death is the only major transformational arc left for the character. (Importantly: This does NOT mean that the character has no flaws left to overcome. For example, he or she may have unresolved regrets (such as in “Gran Torino” (2008).)

2) The protagonist’s death happens as a self-sacrifice that saves and upholds the cause that he or she has championed throughout the film. (For example, as in “I Am Legend” (2007). Or a soldier who is dug in and shooting to cover the helicopter escape of civilians.)

Condtion ‘1’ normally implies old aged, although that need not be strictly the case. A younger protagonist, for example, could be so exceptionally psychologically mature (not just intellectually, but emotionally as well) that he has not much more life evolution left to do. Alternatively, the younger person could have a terminal illness that he is not aware of until Act 2 or 3. (Though, in this case, there should be clues early in Act 1 to hint that his health is not the best – such as inexplicable occasions of coughing, breathlessness, tiredness, etc.)

(In theory, a third youngly-dying protagonist scenario is possible: Where the protagonist’s psychological make up is so limited that it does not permit any further personal evolution apart from death (such as Achilles in “Troy” (2004)). Such characters, however, are harder to empathise with. So this would be risky road to walk down … I wouldn’t recommend it.)

Two good illustrations of Conditions 1 and 2 combined would be the characters of old Bed Kenobi and Gandalf mark I. (Despite the fact they are both Mentors, strictly speaking, rather than true protagonists.)

Firstly, both are at a stage of high psychological maturity early on (much more importantly than simply being old-aged). Neither has much life evolution left for them.

Secondly, when each do decide to really step up to the plate, they do so making a noble self-sacrifice to advance their cause. Their sacrifice is not just about themselves (neither are simply life-weary, for example). And their sacrifice makes a real – not just incidental – difference to their cause. (They both pass their ’batons’ on. Which ultimately leads to victory for their ‘side’.)

So, roughly speaking, as long as your protagonist goes down like Ben Kenobi or Gandalf, your audience should be able to accept your protagonist’s death.

Consider also the cases of King Leonidas in “300” (2007) and Boromir in “Fellowship of the Ring” (2001). Yes, they both die in the end … But who would dare say that their deaths were unmoving, meaningless, or unheroic?

Steven Fernandez