Pick a team, dude: Captain Richard M. Colby was originally sent to kill Kurtz, but when he reached Kurtz's camp, he switched sides. Whatever Kurtz was doing, it really struck a chord with him. Willard replaced him as the army's choice assassin.
When Willard finally arrives at Kurtz's complex, he recognizes a dazed Colby, but Colby doesn't say anything. He's been weirded out for good, and embraced the jungle's madness just like his master.