There are two parts to the ending of Platoon, and they both matter. Big. Time.
Taylor Goes to the Dark Side
First, and most importantly, Taylor kills Barnes. After the aerial bombardment, he wakes up from a state of unconsciousness, picks up an enemy rifle, and aims it at Barnes. Barnes, severely wounded himself, says, "Do it." Taylor obliges and kills him.
For most of the movie, Taylor has remained above the fray, so to speak. He doesn't kill any civilians during the raid on the village, he doesn't rape any women, and even though he wants to kill Barnes, he settles for starting a fight with him in the Underground.
This all changes at the eleventh hour, when Taylor goes from being a morally upright fella to its opposite: a guy who is just as tainted as Barnes, Bunny, and all the rest of the "bad" guys. The symbolism is clear: even the morally upright, levelheaded Chris Taylor is not immune to plagues of a terrible war. When Taylor frags Barnes, he becomes like Barnes (Barnes fragged Elias). Earlier in the film, Rhah mentions that Barnes can't be killed, "the only one who can kill Barnes… is Barnes." Barnes is dead, and Taylor has killed him, which suggests, quite clearly, that Taylor is Barnes, so to speak.
Taylor's killing of Barnes is kind of egregious on another level, too, which brings us to the second part of the ending: Taylor's departure from the battlefield and from Vietnam in a helicopter. In his final narration, he describes himself as a son of two "fathers," Barnes and Elias. If Barnes is one of Taylor's fathers, then Taylor has just killed his father in what amounts to a 20th-century Oedipal conflict.
Sure, this is all very figurative, but it adds another layer to Taylor's final act.
Now, as for Taylor's departure in a helicopter. This, too, is richly symbolic. If Taylor's descent into Vietnam is, essentially, a descent into an updated version of the classical underworld (the realm of the dead). Recall that what greets him on the tarmac: body bags.
Taylor's departure from Vietnam is symbolic departure from the underworld, a small miracle of sorts (he journeys through the realms of death and returns to tell the tale). But even though he returns, he returns a changed, tainted, and haunted man, one aware of his responsibility, to tell his tale and teach others.