Coppola’s Apocalypse Now is inspired by Conrad’s The Heart of Darkness. Coppola adapted the action and characters to his conception of the “insane” war in Vietnam, and the beach party picnic is his addition to the narrative. 
Coppola ensures nothing is quite right in his narrative, especially Lt. Col. Bill Kilgore’s war in Vietnam. As the commander of an Air Cavalry unit Kilgore (the name is a pun) is apt to picnic at the end of a brutal day of action en strafing and killing Vietnamese soldiers and civilians. When the battle is over and the helicopters are secured, the landing zone is set up for a beach picnic party, a bizarre that reminds everyone wistfully of home: “Men are grouped around the fires, eating steaks, hot dogs, hamburgers, drinking beer. It has the bizarre resemblance of some sort of barbarian beach party.”
Content, Kilgore serenades as the picnickers sit beside a cheery fire burning in an oil drum. “Make my meat rare. Rare but not cold,” he calls out to the cook.
The battle and the picnic are a metaphor for the irrationality of war. Kilgore’s personality allows him to kill and picnic without contradiction. Willard explains, “Kilgore had a pretty good day for himself. They choppered in the T-bones and the beer. . . and turned the L.Z. into a beach party. The more they tried to make it like hone, the more they made everybody miss it.”
See Francis Ford Coppola. Apocalypse Now (1979). Screenplay by John Milius, Francis Ford Coppola, Michael Herr; I, by John Milius, Francis Ford Coppola, and Michael Herr (2001)