P-C, having done a little research, I find that the web is (unsuprisingly) very very confused about the quote. About half the sites say it is "Kill" and the other half say "Murder".
So, being the obsessive tyke that I am, I rushed over to Bartleby.com and discovered that Faulkner, if he ever did say either one, was actually quoting someone else.
Yes indeed, this phrase appears to have first appeared in Sir Arthur Quiller-Couch's text, On the Art of Writing, published in 1916.
Here is the paragraph that contains it originally:
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.’
I now return you to your regular Minearverse, already in progress. I accept that I am a tremendous geek obsessed with source citation and should likely be mocked.