Marlow thinks he's being compassionate by lying to Kurtz's intended. But in actual fact, he's unwittingly perpetuating the system of oppression and exploitation which helped to create this colonial Frankenstein's monster. For if people are only palmed off with a sanitized version of Kurtz's last words, how will they ever be able to confront the horrors of colonialism that Kurtz himself recognized all too late? Marlow may have become thoroughly disillusioned with the whole colonial project, but he perpetuates it by romanticizing Kurtz's death.

If Kurtz's intended, the woman who knew him better than anyone else in the world, will never get to know what he became out there in the jungle, then neither will anyone else. It will be Marlow's little secret, something that he will more than likely take to his grave. Yet this information is not his to hold; it must be imparted to others, not just for the sake of Kurtz's intended, but for society as a whole, so people can better understand the damaging effects of colonialism on colonists and indigenous people alike.

In withholding this crucial information, there can be little doubt that Marlow is acting with the best of intentions, wanting to spare the woman who loved Kurtz of the sordid details of this once-great man's demise. But this "noble lie" is also an expression of cowardice on Marlow's part, a representation of his unwillingness to confront his own inner demons, not to mention the system of colonial exploitation he's come to despise.

There are at least two reasons that Marlow chooses to lie to Kurtz' fiancee. The first reason is simple sympathy and the second is that explaining Kurtz dissolution would be nearly impossible. 

Kurtz intended lives under the same misunderstanding that Marlow's aunt does. These women, living with all the comforts of Europe, believe that the imperialist project is a thoroughly moral one. This misconception does not excuse them from actual and economic responsibility for the exploitation undertaken in their name, but it does insulate them from direct, moral responsibility. 

Marlow must feel that Kurtz' intended does not deserve to suffer either from an affliction of reality or from sharing in Kurtz dark realizations. Marlow does not leave the Congo as a messenger of the truth, though he has certainly encountered a difficult truth about human corruption.

Marlow believes he could not have told the truth, something too painful for her to bear.

A natural sympathy might explain why Marlow chooses not to inflict this truth on Kurtz intended. 

Also, how could Marlow explain how Kurtz was found living in the jungle in a hut with heads on stakes surrounding it? Though Marlow understands better than anyone else what happened to Kurtz, the complexity of Kurtz psychological dissolution is difficult to convey. 

Marlow admits to his intimate knowledge of Kurtz while at the same time suggesting the difficulty of truly explaining the man when he speaks with Kurtz intended. 

When she asks Marlow if he had known Kurtz well, he says, “I knew him as well as it is possible for one man to know another.”