During a writing course it is almost inevitable that the instructor will say something like, "As Faulkner said, you have to murder your darlings." Sometimes the quotation is attributed to Mark Twain or Agatha Christie. It means that the writer must go through the work and prune out any phrase that seems particularly fine. I have never understood this advice and believe it to be wrong.
In the first place, it was not Faulkner, nor Twain, nor Christie who said it. (Faulkner's darlings were well-treated and in fact fill volumes.). As near as can be determined the phrase was coined by Arthur Quiller-Crouch in a lecture published as "On the Art of Writing" in 1916. Mr. Q-C was a distinguished scholar and literary gent but his name does not loom large in the annals of fiction. Perhaps he slew too many of his.
What I supposed is meant by the advice is that writers should read over their work and if some phrase seems overwrought, fussy, or inappropriate in the context they should cut it out. But it seems to me that if, when so revising, one comes across a phrase that seems particularly good (a darling) one should, well, leave it be. I mean the point of writing is to write good stuff, and the writer is the judge of what's good in the work. How else can one write?
Of course if one's darlings are actually crap, if one has, in fact, impaired judgment, then one is a bad writer, and no advice is apt to help. The fame of this advice and of many others of the same type exposes the problematic nature of all writing instruction. One could argue that great surgeons or great engineers have profited from great instructors. We may prefer a cardiac surgeon who went to Harvard med school and interned at Mass General--it's not a guarantee of quality, of course, but it's reasonable sieve for patzers. But the same is not at all true for writers. Although many decent writers have been through the schools, many of the best ones arise like mushrooms from obscurity in a manner that remains mysterious. And there is dreadful writing, containing nothing that anyone would call darling, that is wildly successful.
I suppose the only advice that I'd take from the murder your darlings diktat is to read your work over after leaving it for a period and then ask yourself whether, had you paid cash for it, you'd feel cheated. Yeah, thin stuff, but teaching writing is all thin stuff. This leads to a consideration of the obvious case that there is a colossal amount of awful writing put out that lots of people buy, as well as ever greater masses that no one reads. This is a vaster and different topic.