Camus has written an interesting piece on the subject. Meursault, in Camus's "The Stranger," appears to be the quintessential nihilist. A man mentally at odds with the entire universe, he plods through life knowing no certainties and caring for nothing. Despite certain characteristics in common with existentialism, another philosophical school sharing nihilism's rejection of established views, but placing emphasis on responsibility and participation in life, it must be said that because of Meursault's denial of objective truth, and rejection of laws and institutions, he is first and foremost a nihilist.
The first and most significant pointer to Meursault's nihilistic tendencies is his denial of any objective basis for truth. This fairly hard to ascertain, because due to the ambiguous nature of the narrative, very little is actually established as 'true'. But it is nonetheless an obvious characteristic of Meursault, as can be established through his attitude toward his own life. After all, that life itself exists, and is purposeful, is one thing that nearly everyone considers to be true. But Meursault treats seemingly important decisions with great indifference, and indeed takes little or no interest in anything. If nothing exists and it is impossible to know anything (because anything you think you know is completely subjective), why bother doing anything? Early in the book , when his mistress Marie asked him about marriage, he said he would marry her, but that he didn't love her, and more significantly, that "... it didn't make any difference to me." He had no emotional attachment to her, or any person for that matter, and he only associated with her because of his physical needs. There is no objectively provable reason to do anything, and people may or may not exist. So why should he care about anyone?
Then, after another, even more serious act, the senseless murder of a friend's enemy, when asked if he regretted the action, he said merely "I felt kind of annoyed." People don't matter, dead or alive. He could have killed someone, or he could have done nothing, or neither of them could have ever existed. There is no reason for him to take any interest in it, from his standpoint. The event was no more than an annoyance. Naturally he takes a similar attitude towards his own life when it is endangered. "Nothing, nothing mattered, and I knew why. So did he. Throughout the whole absurd life I'd lived, a dark wind had been rising toward me from somewhere deep in my future, across years that were still to come, and as it passed, this wind leveled whatever was offered to me at the time, in years no more real than the ones I was living. What did other people's deaths or a mother's love matter to me; what did his God or the lives people choose or the fate they think they elect matter to me when we're all elected by the same fate, me and billions of privileged people like him who also called themselves my brothers."
This is the complete acknowledgment of nihilistic ideas. He totally resigns himself to the irrelevance of his own existence. He says that nothing matters, that life, death, and love are all totally meaningless and potentially nonexistent things. However, does not this evidence also point to Meursault as an existentialist? Existentialists, at least non-religious ones, maintain that the universe is meaningless, and human existence serves no purpose. From this standpoint would not his actions and attitude towards most things have been roughly similar? He doesn't care for people because their existence is pointless. He commits murder and then shows no emotion over it, because again the life served no purpose, and furthermore, as an existentialists, he believed in his own absolute free will and rejected all morals. Additionally, in the above quote, Meursault hints about his ideas of fate, as though the universe was behaving as an uncaring, irrational entity, as existentialists believe it does. Ultimately Meursault cannot be an existentialist, however. Because while existentialists believe that life is meaningless, they also emphasize the responsibility of people for forming their nature, and they emphasize the importance of personal decisions.