In Heart of Darkness, why does Marlow describe Kurtz as a "universal genius"?

Marlow calls Kurtz a universal genius because of his take on humanity and his talents in a wide variety of subjects.
Marlow has been interested in Kurtz since he first heard about the man. He wasn't sure why Kurtz had so many people discussing him; he didn't know what the man's motives were or why he made the choices he did. It was difficult to separate the truth from the fabrications. Once Marlow met Kurtz, however, he was able to see more of the man himself.
Kurtz is never completely defined by Marlow. Marlow isn't sure what his primary interest and talent was because he was good at writing and at painting. Marlow says, "To this day I am unable to say what was Kurtz’s profession, whether he ever had any—which was the greatest of his talents." He hears of Kurtz as an excellent musician, a painter, and a potential politician. Kurtz's talents encompass a variety of spheres.
Marlow goes through a variety of feelings toward Kurtz but in the end, he believes the man's final words show that Kurtz had a greater understanding of himself and the world. This is something that Marlow acknowledges and admires.

Marlow mentions that Kurtz is a "universal genius" in the conversation that he has with Kurtz's cousin after Kurtz has died. Marlow is in the "sepulchral city," where the company's management is based, and Marlow says of Kurtz, "He was a universal genius—on that point I agreed with the old chap." Kurtz's cousin describes Kurtz as a musical genius, and Marlow realizes that he does not even know what Kurtz's profession was, though Kurtz had interests in writing, painting, and journalism.
In addition to possessing this broad range of talents, Kurtz seems like a "universal genius" because of his reported last words, which were, "The horror! The horror!" Marlow says of Kurtz's statement, "After all, this was the expression of some sort of belief; it had candor, it had conviction, it had a vibrating note of revolt in its whisper, it had the appalling face of a glimpsed truth." In other words, Marlow admires Kurtz because Kurtz, in his last moment of life, dared to make a bold admission and assertion that what he had witnessed of the European involvement in Africa was wrong. Marlow refers to Kurtz's statement as a "moral victory" that has been earned through defeat. That is why, Marlow says, he remains loyal to Kurtz and his memory. In the end, Kurtz admitted that what he had seen and done were travesties of enlightenment.