Nancy Friedman has posted a piece on the use of coup de grâce. She suggests reasonably that when we adopt phrases in form from other languages we would do well to keep content in mind. It reminds me of the use of mano a mano to mean man to man when the Spanish phrase really means hand to hand. There's also mano y mano out there which means hand and hand.
Her advisement is not concerned so much with the literal meaning, nor does she rail against the evils of language change. Her point is reasonable because her concern is that the intended message of the phrase changes so much as to risk an undesired effect. And a merciful strike on a delicious dessert can sound a bit like tasty euthanasia.
The problem with coup de grâce is that grâce reminds us of the everyday meaning of English "grace": "elegance," "attractiveness," "charm." But "grace" and grâce also have theological meanings of "mercy" and "thanksgiving" (which English retains in expressions like "by the grace of God" and "the grace before meals"). A coup de grâce relies on the latter meaning: it's a merciful end to suffering.
I left a comment in defense not of semantic shift in general but of the possible appropriateness of the connotations. (Tho I do of course like to defend language change and semantic shift.)
There's quite a tradition of the use of images of death and murder in connection with very positive and very enjoyable events. A comedian kills or murders the audience. It was so funny I died. You slay me. That musician is destroying that solo. Those sweets are deadly. Death by chocolate (which I mentioned in my comment). A dish to die for. And another French phrase: la petite mort -- most enjoyable.
Coup de grâce is often used to indicate a final bit of flair or decoration that tops it all off. It's a bit like the finishing touches on a work of art (which the OED takes back to a literal last touch of a paintbrush). And I can accept that the original sense of a blow to end suffering might occasionally interfere with the sense of beauty or appreciation. But the idea of skill and artistry combined with style and elegance does a lot of skating back and forth between art and warfare and cooking and art and cooking and warfare and … I guess that's all of them.
Art as used to describe the precision of violence.
Remember the line from The Princess Bride?
Inigo Montoya: Kill me quickly.
Westley: I would sooner destroy a stained glass window than an artist like yourself.
Killing as used to describe power of elegance.
From a review of Sex and the City: The Movie:
And the coup de grace: a pair of gorgeous vintage Italian black heels that a friend gave me last week.
Violence as used to describe the effect of good flavour.
From a blog post:
this pizza made from scratch with fontina cheese and homemade tomato sauce and dough. It was so good I thought I would die.
But for some reason it's hard to find descriptions of assassins and warriors with skills full of flavor and decadently sweet. He smote and slew his foe with such spice and savoury succulence... It misses the mark.