There’s a commonplace, a phrase writers toss off rather casually to each other, in various versions: kill your darlings, or murder your darlings. “Murder your darlings” is a phrase that’s often used as if everyone knows the reference and the meaning, which of course, is not always the case.
To begin with, let me plead that you have been told of one or two things which Style is not; which have little or nothing to do with Style, though sometimes vulgarly mistaken for it. Style, for example, is not—can never be—extraneous Ornament. You remember, may be, the Persian lover whom I quoted to you out of Newman: how to convey his passion he sought a professional letter-writer and purchased a vocabulary charged with ornament, wherewith to attract the fair one as with a basket of jewels. Well, in this extraneous, professional, purchased ornamentation, you have something which Style is not: and if you here require a practical rule of me, I will present you with this: “Whenever you feel an impulse to perpetrate a piece of exceptionally fine writing, obey it—whole-heartedly—and delete it before sending your manuscript to press. Murder your darlings.”
Quiller-Couch is talking about style, and, more importantly, about controlling style, rather than letting your style control what and how, and why, you write. The idea behind “murder your darlings” is that writers should murder that text that they are in love with for purposes of display, for ostentatious “see how I did that? Isn’t it clever?” reactions from the reader.
Style should serve the purpose of the text, not the writer’s ego.
The core principle behind “murder your darlings” is that if the text doesn’t serve to drive your story forward (or support your argument), if its purpose is purely ornament, then kill it.