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"I fancy I had some vague notion of falling upon him and giving him a drubbing. I don't know. I had some imbecile thoughts. The knitting old woman with the cat obtruded herself upon my memory as a most improper person to be sitting at the other end of such an affair. I saw a row of pilgrims squirting lead in the air out of Winchesters held to the hip. I thought I would never get back to the steamer, and imagined myself living alone and unarmed in the woods to an advanced age. Such silly things—you know." (3.26)
When Marlow goes chasing after Kurtz, he is confused and has evil thoughts like beating him or "giving him a drubbing" when he finds him. He is confused and certain images burst into his mind. Marlow is concerned mainly with an inevitable sense of catastrophe (which is why he thinks of the old woman who represents Fate) and fear (represented by the pilgrims shooting blindly from their hips).
"There was nothing either above or below him, and I knew it. He [Kurtz] had kicked himself loose of the earth. Confound the man! He had kicked the very earth to pieces. He was alone, and I before him did not know whether I stood on the ground or floated in the air." (3.29)
Kurtz has "kicked himself loose" of all things that humans know, which means that he has no set of morals and no definitions of good or evil anymore. Does that make him crazy? And does that mean that "crazy" is just relative to our expectations?
"I've been telling you what we said - repeating the phrases we pronounced—but what's the good? They were common everyday words - the familiar, vague sounds exchanged on every waking day of life. But what of that? They had behind them, to my mind, the terrific suggestiveness of words heard in dreams, of phrases spoken in nightmares." (3.29)
By this point, Marlow is basically living in a waking nightmare. He seems to be at a point where he can't even tell what's real.