[Hookbill the Goomba]

Boromir's character arc goes through numerous changes throughout Tolkien's writing of The Lord of the Rings...

Originally Posted by Kuruharan

Boromir seems to my mind to be a character who was created just to die.

... This may be how it seems at first, but the development of the character has some interesting twists and turns. According to the outline of 'The Story Foreseen from Moria' (Chapter XI from History of Middle Earth Volume 7 - The Treason of Isengard), Boromir survived the breaking of the Fellowship and travelled with Aragorn.

Originally Posted by HoME VII: The Treason of Isengard; Chapter 11

"Boromir and Aragorn (who notes a change in Boromir - who is keen to break off the chase and go home) reach Minas Tirith, which is besieged by Sauron except at back. ? [...] The Lord of Minas Tirith slain and they choose Aragorn. Boromir deserts and sneaks off to Saruman to get his help in becoming Lord of Minas Tirith."

The section is crossed out and replaced with the tale of Legolas and Gimli being captured, which is also crossed out.

There are further plans, still keeping Boromir alive. Eventually, however, Tolkien decides;

Originally Posted by HoME VII: The Treason of Isengard; Chapter 11

"What about Boromir? Does he repent? [Written later in margin: No - slain by Aragorn]"

As Tolkien comes to write the Breaking of the Fellowship he outlines the orc attack and Boromir's death for the first time.

Boromir's death is an event that seems to dawn on Tolkien as he discovers the depth of his treachery. Alas, I find nothing in the letters on Tolkien's decision to kill him at Amon Hen, rather than by Aragorn at some later point. I wonder if it was simply a narrative tool in the beginning.
But one could read something into it with regard to a comment I happened across a moment ago. With regards to Faramir.

Originally Posted by The Letters of J.R.R. Tolkien #65

"Faramir [...] is holding up the catastrophe by a lot of stuff about the history of Gondor and Rohan (with some very sound reflections no doubt on martial glory and true glory)"

The fact that Boromir's death happens "off screen", so to speak, is interesting to me and I wonder if it is part of one of Tolkiens themes of 'martial glory vs. true glory'. In his death, Boromir achieves a sort of martial glory, as he as done in his life. A heroic death against many foes. And yet at the same time, he dies defending the Hobbits, repenting of his fall to the Ring's influence. Does he, therefore, almost reach 'true glory' beyond simple martial glory?

The Lord of the Rings does seem to stand in contrast to The Silmarillion, in my mind, on this point. Where the latter is filled with martial glory (and the terrible consequences of it), The former shows the victory of the little Hobbits, where all the assembled armies of Middle Earth failed. So with Boromir's death, perhaps, we are seeing this shift. We see the warrior falling not to defend the city, not to fight Sauron himself, not surrounded by banners and songs, but defending hobbits.