What is the climax of "Heart of Darkness"? I think it's when Kurtz dies.
I think you are correct. The climax comes when Kurtz dies---leaving the mystery to what he means by "the horror, the horror." Kurtz had "stepped over the edge" while Marlow has stepped back from the madness that consumed Kurtz. The rest of the story is simply the resolution of Marlow's search and culmination in allowing Kurtz's intended to remain unaware of Kurtz's insanity.