What is the main theme of The Big Sleep by Raymond Chandler?

The main theme of The Big Sleep is
the immorality and evil rampant in the big city which Philip Marlowe encounters
everywhere while conducting a private investigation. "The big sleep" is a synonym for
death. There are a number of murders in Chandler's novel--first Arthur Gwynn Geiger,
then Joe Brody, then Harry Jones. Then Marlowe kills Canino and finally discovers that
Carmen Sternwood had killed Rusty Regan. Below are some excerpts to support the
theme:

"Photos and letterpress were alike of an
indescribable filth. The book was not new. Dates were stamped on the front endpaper, in
and out dates. A rent book. A lending library of elaborate smut."  Ch.
5

"She was wearing a pair of long jade earrings. They were
nice earrings and had probably cost a couple of hundred dollars. She wasn't wearing
anything else."  Ch. 7

"You think he's just a gambler. I
think he's a pornographer, a blackmailer, a hot car broker, a killer by remote control,
and a suborner of crooked cops. He's whatever looks good to him, whatever has the
cabbage pinned to it.'"  Ch. 28