One of the things I noticed was that Heart of Darkness and The Stranger got us to similar places.
I like this passage:
Pg. 106 - "Droll thing life is -- that mysterious arrangement of merciless logic for a futile purpose. The most you can hope from it is some knowledge of yourself -- that comes too late -- a crop of unextinguishable regrets. I have wrestled with death. It is the most unexciting contest you can imagine. It takes place in an impalpable greyness, with nothing underfoot, with nothing around, without spectators, without clamour, without glory, without the great desire of victory, without the great fear of defeat, in a sickly atmosphere of tepid skepticism, without much belief in your own right, and still less in that of your adversary. If such is the form of ultimate wisdom, then life is a greater riddle that some of us think it to be."
These ideas and Heart of Darkness in general reminded me of existentialism. There's the characterization of life as futile, similar in some ways to the idea of absurd as used by Camus. A lot of systems get rejected -- European morality is shown to be a mockery, success/ambition/pride are debunked by Kurtz's life... everything seems pretty vague and formless, like the description of death. As he returns to the sepulchral city, Marlowe sees everyone else's lives as petty and worthless, concerned with trivial things, because he has been enlightened in a similar way that Meursault was.
But I think that there is a subtle difference in the way Meursault and Marlowe view existence. For Meursault, the world wasn't necessarily good or bad, just indifferent. Neutral. Life is what you make it. Marlowe, on the other hand, returned from his trip with a feeling of disgust for human nature and a sense of futility regarding life. His view was negative. Life is a horror. This might explain why Meursault turned out (arguably) happy, whereas Marlowe ended the novel in a really bad mood.