Whe something is didactive, or didactic, it is instructive or teaches a lesson. The word also implies a heavy-handedness in this instruction. The lesson mentioned in the previous post is clear. But I also think that a major didactic lesson in this novella is that one must guard one's heart against darkness, that unless the heart is guarded, the darkness that is innately within humankind will manifest. Kurtz is a perfect example of what can become of an unchecked heart. In the wilderness where there is no law or civilazation to keep him in check, Kurtz's dark nature overtakes him, and he develops a god comlex, believing it within his rights to savagely murder anyone who does not worship him. He even goes so far as to participate in human sacrifices in his honor, possibly eating those sacrificed to him. His home is surrounded by shrunken heads, all but two of which face his windows. This shows that for the most part, these heads are there for his own sick pleasure rather than as a warning to intruders, otherwise, they would have faced outward. Marlow sees the potential for this darkness within himself:
“The earth seemed unearthly. We are accustomed to look upon the shackled form of a conquered monster, but there—there you could look at a thing monstrous and free. It was unearthly, and the men were—No, they were not inhuman. Well, you know, that was the worst of it—this suspicion of their not being inhuman. It would come slowly to one. They howled, and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity—like yours—the thought of your remote kinship with this wild and passionate uproar. Ugly. Yes, it was ugly enough; but if you were man enough you would admit to yourself that there was in you just the faintest trace of a response to the terrible frankness of that noise, a dim suspicion of there being a meaning in it which you—you so remote from the night of first ages—could comprehend. And why not? The mind of man is capable of anything—because everything is in it, all the past as well as all the future. What was there after all? Joy, fear, sorrow, devotion, valor, rage—who can tell?—but truth— truth stripped of its cloak of time. Let the fool gape and shudder—the man knows, and can look on without a wink. But he must at least be as much of a man as these on the shore. He must meet that truth with his own true stuff—with his own inborn strength. Principles? Principles won't do. Acquisitions, clothes, pretty rags—rags that would fly off at the first good shake. No; you want a deliberate belief. An appeal to me in this fiendish row—is there? Very well; I hear; I admit, but I have a voice, too, and for good or evil mine is the speech that cannot be silenced. Of course, a fool, what with sheer fright and fine sentiments, is always safe. Who's that grunting? You wonder I didn't go ashore for a howl and a dance? Well, no—I didn’t. Fine sentiments, you say? Fine sentiments, be hanged! I had no time.
Here Marlow shows the wildness of humanity, and his own desire at times to join it. It isn't humanity that constrains him, it is duty. Had he time (as Kurtz had had) he likely would have become as dark and wild as Africa, and as dark and wild as Kurtz. So the lesson is that there is the potential for evil in even the most civilized man--that perhaps the civilized, not having been exposed to real freedom, are even more suseptible to it.
It shows how an ideology can be used to justify genocide and how the language is used to manipulate people. Africans are called enemies, savages, criminals and rebels and thus dehumanised so that the crimes against humanity (genocide, robbery, torture, killing, enslaving of the natives) could be justified. It also shows us "how much out of touch with the truth" anyone of us can be. In addition, it portrays in great detail the consequences of greed and sin on a man's soul.