The ending of The Big Sleep doesn't have the same wow factor of a Sherlock Holmes novel, where Holmes ties together all the loose threads in a virtuoso display of intellectual superiority over his adversaries. On the contrary, there are still plenty of loose ends in The Big Sleep, most notably what happened to Owen Taylor. Was he murdered or did he commit suicide?
Chandler hated the expectation that all detective stories needed to tie up every loose end. He also didn't want to have perfectly rational explanations for human behavior because in real life nothing is tied up neatly with a big pretty bow. Life is messy and sometimes things are unexplainable.
Sure, in the final pages of The Big Sleep, Marlowe has solved most of the case. He figures out who killed Rusty and how his murder was covered up. But in many ways the ending of the novel leaves things unresolved and open-ended. Mars doesn't get any prison time, even though he was behind many of the murders and crimes. The secret of the Sternwood family won't be made public, and Carmen will (in theory) be cured, not punished. Vivian also won't have to pay for covering up the truth. And the General may have been spared the painful knowledge of Regan's murder, but he'll also die without ever learning the truth.
All of this raises the question of how successfully Marlowe has performed his job. Is it even possible for Marlowe to ensure that justice has been met in such a depraved world? Has Marlowe himself been able to keep his own hands clean while trying to dig up the truth behind the Sternwoods' sordid family history? At the end of the novel, Marlowe contemplates the events that have unfolded, and we finally learn the meaning behind the novel's title
What did it matter where you lay once you were dead? In a dirty sump or in a marble tower on top of a high hill? You were dead, you were sleeping the big sleep, you were not bothered by things like that. Oil and water were the same as wind and air to you. You just slept the big sleep, not caring about the nastiness of how you died or where you fell. Me, I was part of the nastiness now. Far more a part of it than Rusty Regan was. But the old man didn't have to be. He could lie quiet in his canopied bed, with his bloodless hands folded on the sheet, waiting. His heart was a brief, uncertain murmur. His thoughts were as gray as ashes. And in a little while he too, like Rusty Regan, would be sleeping the big sleep. (32.77)
Okay, so there's a lot that Marlowe is saying in this moment of reflection. Keep in mind that the guy's at his most cynical right now. He has just left the Sternwood mansion knowing that no one will pay the price for the crimes that were committed. Justice shmustice.
So what's his takeaway from all this? Marlowe thinks that the only way to escape the nastiness of life is through death. In fact, only the character of Regan (who we never get to meet) is left untarnished since he's already dead when the novel begins. He doesn't have to care about the "dirtiness" of life. Similarly, when General Sternwood dies, he'll also be at peace. Marlowe realizes that as long as he is still alive, he'll always be mixed up in the "nastiness" of the Sternwood case. Ironically, Marlowe's moment of supreme pessimism is also a moment of optimism in that death is portrayed here not as the gruesome murders we've witnessed previously, but as a gentle sleep.
We leave Marlowe as he attempts to come to grips with what has happened, to make sense out of everything. But he can only reach a partial understanding. As a detective who does his best to act as a modern-day knight, Marlowe can only be so successful in his quest for truth. He's able to solve some mysteries and partially expose certain crimes, but he ultimately realizes how little control he actually has over things. In the seedy underbelly of Los Angeles, Marlowe must resign himself to the fact that the immoral standards of modern society will remain.