The phrase "hollow at the core" suggests what?

The full quote is:

"But the wilderness had found him out early, and had taken on him a terrible vengeance for the fantastic invasion. I think it had whispered to him things about himself which he did not know, things of which he had no conception till he took counsel with this great solitude—and the whisper had proved irresistibly fascinating. It echoed loudly within him because he was hollow at the core ." (ch 3)

This quote speaks to the "darkness: that envelopes Kurtz in the jungle. Kurtz is drained of whatever he once knew and is left an empty vessel at the mercy of the shadows that now possess him. Kurtz is no longer the man he once was: he is "hollow at the core."