Apocalypse now

During the Vietnam War, the American military secret services entrusted Captain Willard with the task of finding and executing Colonel Kurtz, whose methods were deemed "unhealthy". This one, established beyond the border with Cambodia, took the head of a group of natives and carries out operations against the enemy with a terrifying savagery. Using a patroller placed at his disposal, as well as his crew, Willard must go up the river to the depths of the jungle to eliminate the officer. During this trip, he discovers, while studying the file of Kurtz, a man very different than he imagined. How could this officer with an exemplary career become the bloodthirsty madman that we describe to him?