What is Marlow's lie?

Marlow lies to Kurtz's intended wife about his actions and death. He believed the truth would hurt her, that teling her the man she believed to be so good was really evil would have caused her distress and grief that she didn't need to bear. Was he justified? Well, to answer that question we need to look at the fact that Marlow hated lying and ony told two throughout the course of the story. To save a living woman grief.... yes, I think he was justified.