This might help with the OP's question:
The trip up the river led by Capt. Willard exposes him to two extreme viewpoints of war represented by the two colonels he encounters on his long journey, both of whose names start with 'K,' which is no accident:
COL. KILGORE (Duvall) is a romantic who embraces war as a lifestyle and even feeds off it, i.e. glorifies it. The fact that he's a romantic can be observed in the air-raid on the village where he literally plays Wagner as a prologue. He feeds off the war to the extent that he "loves the smell of napalm in the morning." War is just another day to him so why not go surfing? Since he lives off of the war there's no way it can kill him or even give him a scratch. Kilgore naturally has the support of the top brass because he's part of the system and plays the game of war.
COL. KURTZ (Brando), by contrast, sees through this hypocrisy. He realizes that being in a state of war is humanity gone mad. It's horror itself and therefore must be ended through the quickest means possible, whatever cost. He refuses to play the game of war as he expertly takes out double agents, etc. Of course the brass can't have this so they put out a hit on Kurtz via Willard. The existential Kurtz becomes increasingly disillusioned -- even crazy -- after jumping ship from the system and now has no sanctuary. Death is the only way out. His consolation is that Willard will tell his son the truth.
Was Kurtz really insane or just enlightened, even genius? General Corman informs Willard: "He's out there operating without any decent restraint, totally beyond the pale of any acceptable human conduct. And he is still in the field commanding troops." Yet Kurtz was accomplishing what the US military couldn't or wouldn't do because of political complications and niceties.