"I am reality," a drunk Sergeant Barnes exclaims shortly after he's just finished killing Elias in the jungle.
But you know what? He's kinda right. He is reality—the reality that the Vietnam War featured lots of hardened, violent, unforgiving men just like Barnes, guys who wouldn't hesitate to kill innocent people, or even their fellow soldiers. But let's backtrack here for a second.
What Is Barnes, Exactly?
He's a legend, that's for sure, and his fellow troops know it and believe it, even the detractors (like Taylor, like Rhah). Barnes is a very mythological figure, a guy of nearly supernatural strength and endurance. His heavily scarred face suggests a guy who's been around the battle block more than a few times, and the fact that he's been shot 7 times, as Rhah tells us, and is still alive makes him seem immortal: "Barnes ain't meant to die. The only thing that can kill Barnes is Barnes" (Rhah to Taylor).
Barnes' strength, endurance, is matched only by his deep knowledge of the war, the enemy, and the terrain which makes him the platoon's natural leader. Sure, Lieutenant Wolfe is nominally in charge, but Barnes really runs the show. He has the experience, the loyalty of the troops, and the skill set to keep them safe. He spots the bunker that Taylor stumbles on before anybody else, and he ferrets out the truth in the village. In one particularly famous scene Lt. Wolfe calls in the wrong coordinates for an air strike, ultimately hurting some of the men. Naturally, Barnes corrects the situation… right before he kills Elias.
See, Barnes is a great soldier, and well-respected one, but he's also a loose cannon. Vietnam is his war, as he says to Elias, and he'll do whatever it takes to protect himself and his men, even if that means offing a naïve "crusader" like Elias. Elias is planning to file a report about Barnes' brutal execution of an innocent Vietnamese woman (one of his more deplorable actions), and Elias takes Barnes down in a fight. Barnes, being the alpha male that he is, and obeying the laws of the jungle, so to speak, by eliminating the potential threat and rival. Looked at from that lens, Barnes seems like a guy who does whatever he has to survive.
The problem is he has no moral compass whatsoever. He's about to kill a little girl at one point (that's when Elias hits him in the face with the butt of his rifle), he kills Elias, he cuts Taylor's face with a knife (he seemed poised to kill him, too), he brutally kills enemy soldiers in the final firefight, and he's about to kill Taylor when the air strike knocks them both unconscious. In a lot ways Barnes is a figure of pure instinct—the instinct to survive—and killing is often a part of that will to survive.
It makes sense, thematically then, that Barnes dies. Just as there's no place in the Platoon universe for a morally perfect "crusader" like Elias (he's too good for that world), there's equally no place for a savage animal of pure instinct and murder like Barnes. Both of Taylor's "fathers" die, and as much as we hate it admit it, Taylor has a little bit of both of them in him, Elias and Barnes.