When Willard hacks Kurtz to death, it syncs up with the scene of Kurtz's followers ritualistically killing a water buffalo outside—just like Pink Floyd's Dark Side of the Moon syncs up with The Wizard of Oz when you play them both at the same time. (It's true—they do.)

Not only did the water buffalo sacrifice win the American Humane Association's stern disapproval—since an animal was harmed (killed) in the making of this movie—but it symbolizes the idea that Kurtz's own death is a ritual sacrifice. Willard says that the jungle itself wanted Kurtz dead.

According to Francis Ford Coppola, he intended the scene to depict Willard killing the darkest aspect of himself, represented by Kurtz. Religious animal sacrifices are typically done to purge the people of their guilt, and we're meant to think that this is what's happening to Willard.

Afterward, Willard's faced with the choice of becoming the new king of Kurtz's followers, the same way the killer of the king in ancient pagan rituals would become the new king, according to the 19th-century book The Golden Bough. (A copy of The Golden Bough is visible among Kurtz's belongings.)
(Source)

Willard declines the deity job. Who needs the pressure, and anyway, it doesn't come with health insurance or paid vacation.