Eating somebody's heart is a pretty nasty idea. For a lady like Beatrice who has so far been pretty ladylike, it's astonishing. You picture her, blood dribbling down her chin as she munches aorta over the opened corpse of Claudio. I can't think of a single line anywhere else in English literature that changes the tone so quickly*. It's like the zero-zero ejector seat, which a pilot can use even when the plane is stationary on the ground. He can sit in the cockpit drowsily humming a tune to himself and watching the flowers grow, then touch a button and a bomb explodes beneath his seat and he is sent hurtling into the sky.
Eating somebody's heart is pretty hideous, but the line wouldn't have the same effect, we would not picture it so clearly, were it not in the marketplace. Because it's in the marketplace, we see the cannibalism. Because it's in the marketplace, we realise that this is not a figure of speech but a plan, a plan with a location.
Shakespeare was all technique. Every good idea he had he used again. After Hamlet has killed Polonius, Claudius says to Laertes (Polonius' son):
Hamlet comes back: what would you undertake,
To show yourself your father's son in deed
More than in words?
And Laertes replies
To cut his throat i' the church.
I hear that line and think to myself: "Done that one before, Will. Slightly better the first time, but good effort, Will, good effort." It works: violence is twice as vivid with a location attached.
Slitting people's throats is a commonplace threat. I have heard respectable and peaceable matrons complain of some minor annoyance and add "Oh I could slit his throat." They don't mean it and I don't picture it. It is a figure of speech, no more to be taken literally than the strange evolution implicit in "Son of a bitch", or the eternal agony and torture wished for in "Damn him."
But add a location, preferably an inappropriate one like a church or a marketplace, and the image revives, the threat is precise, we see the knife cutting the skin, or the teeth breaking the ventricles.
And all by simply appending to the threat a single, simple clause.
"I'll kill you." - Not that frightening.
"I'll kill you on Tuesday." - Ooh.
A technique, dear reader: a technique to be learned and used.
While we are on the subject of hearts, heart strings (which are so often tugged) are real. Medical fellows call them chordae tendineae, because medical fellows will do anything to avoid speaking English. If anyone ever did tug on your heart strings, you would die. They might also get stuck between Beatrice's teeth.
Label on the upper right
*Although you are welcome to nominate a rival in the comments.
P.S. I got glared at furiously by a passing lady on Saturday just because I happened to observe to a friend that the best way to a girl's heart was keyhole surgery.