I think the knowledge came to him at last - only at the very last. 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.
Conrad, Joseph. Heart of Darkness. London: Penguin, 2000, p. 95.
The Dark Knight reminds me of Heart of Darkness. This book and movie aren't really anything alike, but both Kurtz and the Joker are terrible people, and in that sense they are the same.
This passage sums up the whole book, for me, and I think about it.