You are about to hear a long, strange tale, full of myths, men, and… rewrites.
The Weird Original
The origins of the Platoon screenplay go back to 1969, when a young Oliver Stone first started to grapple with the horrors he experienced during his own 15-month service in Vietnam. The result? A strange, myth-laden screenplay simply titled Break.
This early version describes a young soldier who goes off to Vietnam, dies, and then ends up in the Egyptian underworld. It was all very "mythic," according to Stone, a product of the fact that he wasn't ready to "deal with Vietnam […] in a completely realistic way." The title may have been inspired by The Doors' song "Break on Through." The early script incorporated lots of their music, and Stone even sent a copy to Jim Morrison, whom he wanted to star in the film (Morrison had the script in his apartment in Paris when he died [source]).
In 1976, Stone wrote another screenplay about Vietnam called The Platoon, borrowing some of the characters from Break (who were based on people Stone knew in Vietnam) but eliminating the whole Doors thing and the heavy mythology. The Egyptian underworld of Break was replaced by the more realistic underworlds of the camp—the underground cellar where the "heads" (the potheads) hang out and smoke pot—and Vietnam itself, which is presented to the audience as an underworld (Taylor's descent into Vietnam is a descent into the world of the dead, symbolized at the beginning by the body bags that greet him on the tarmac).
While the script written in 1976 is less mystical, the mythological element was still there, if transmuted. For example, Stone has talked about how the rivalry between Elias and Barnes, in addition to being based on two guys he once knew, was also based on characters from Homer's Iliad. According to Stone, it was all about "The angry Achilles versus the conscience-stricken Hector fighting for a lost cause on the dusty plains of Troy" (source). In addition to this, ahem, Iliadic tinge, there are also Oedipal overtones. Near the end of the film, Taylor describes himself as a son of both Barnes and Elias. We already know at that point that Taylor has killed Barnes, which means he's killed one his fathers—just like Oedipus.
Approaching Final Draft
All of these elements are in the final film. The biggest, and most important difference, between the 1976 first draft of The Platoon and the revision Stone made in 1984 in preparation for the making of the film was not the name change (it was called just Platoon by then), but Taylor's killing of Barnes. In the early version from 1974 Taylor leaves Vietnam without killing Barnes, his moral compass still pointing "north," so to speak.
By 1984, Stone had a change of heart. It's deeply important that Taylor leaves Vietnam tainted, that he is unable to avoid partaking in the very horrors his earlier, more naïve self, had vociferously fought against. The symbolic point is clear: war is bad for everyone, and there is no way to remain decent in war without dying (Elias stays decent, and he dies).