The Kurtz and Willard relationship

I watched the 1979 FRANCIS FORD COPPOLA epic "APOCALYPSE NOW" on DVD this weekend.
I was particularly interested in the relationship between the characters of Capt. Willard (Martin Sheen) and his upriver target, Col. Kurtz (Marlon Brando).
In his book about "Apocalypse Now," Peter Cowie explores the pair of characters who meet in the film's final act.
"The character of Willard, the army captain who toils up-river to terminate Colonel Kurtz and his renegade command, was based on Fred Rexer, whom (scriptwriter John) Milius had met at a gun show soon after completing 'The Wind and the Lion.' Rexer had been a Green Beret in Laos and had taken part in the Phoenix programme to subdue VC influence in the villages. Rexer had actually experienced the scene recounted by Kurtz in the film, where the arms of children are hacked off by the Vietcong. Sensing the symbiosis between Kurtz and Willard, (director Francis Ford) Coppola wrote in one of his earliest notes about the film in prospect, 'if (Willard) can accept Kurtz, then he can accept himself.' By comprehending Kurtz he also enters into some strange alliance with him."
I watched with interest as the two interacted, and Kurtz instructed Willard to take his story back to civilization.