Sparing the Aces

"I would as soon destroy a stained glass window as an artist like yourself."

—The Man in Black, The Princess Bride

Alice is a once-in-multiple-lifetimes genius at something. What it is doesn't matter - a brilliant general, a mathematician, a painter. Bob has a very good reason to want to kill her and is in a position where he can easily do so. He decides against it despite the fact that he could safely get away with it.

Why? Because the life of someone so brilliant has intrinsic value, and he doesn't want to deprive the world of it. Some overlap with Kill Me Now or Forever Stay Your Hand, and a sister trope to Uniqueness Value. While that trope demands uniqueness, this one required exceptional genius at something in particular—if there happened to be five brilliant physicists, the character acting out this trope would still have as much trouble killing them as if there were only one at that level.

Frequently, the "genius" in question becomes a Morality Pet. A subtrope of You Will Be Spared.

Examples of Sparing the Aces include:

Anime and Manga

Played with in the 'ant' arc of Hunter X Hunter. First, all the humans in the country the King has taken over are to be tested to destruction in such a way that the most potentially powerful can be converted into the superpowered mindslave footsoldiers of his world conquest. Then to keep him busy for a few months while they get this plan into operation, his bodyguards start bringing the most accomplished strategy gamers they can find; once he beats the national chess champion, the man is killed and eaten, etcetra. Then he can't best this blind girl with a runny nose, Komugi, and starts to get obsessed with doing so. That's all stage one.
- Stage two, she starts visibly improving in response to his challenge, and he comes to the thoughtful realization that human beings have an endless and mysterious potential to improve, and that child that he ate on the way to the palace might have had the potential to exceed him, the King, in some particular way. His bodyguards are at this point in the speech freaking out at what looks like an impending Heel Face Turn...and then we get a giant Slasher Smile and the declaration that if they are all that, and he can kill any of them, that makes him even more supremely awesome than he'd realized.
- Stage three, he develops even more respect for Komugi and some form of emotional maturity (the guy is three months old), which combined with his appreciation for the skill and determination of the Hunter strike force, or at least Netero, prompts him to offer very reasonable terms, which are not accepted. He eventually dies of radiation poisoning inflicted by Netero's dying act, while playing against Komugi. He never did beat her.
- The King's arrival at any kind of civilized interaction with humans was thus built on a foundation of this trope, via Komugi and Netero.

Film

The Princess Bride: The Man in Black declines to kill his Worthy Opponent Inigo Montoya because the latter is an extremely gifted swordsman who has made swordfighting his life's study in order to be able to one day avenge the man who killed his father.
In Hellboy II the Golden Army, Prince Nuada pulls this gambit to try to turn Hellboy over to his side. It rings just a little bit false, because Nuada himself unleashed the creature in question as a weapon.

Nuada: What are you waiting for? This is what you want, isn't it? Look at it. The last of its kind. Like you and I. If you destroy it, the world will never see its kind again...

In The Silence of the Lambs, Hannibal Lecter spares agent Starling for precisely this reason.

Clarice Starling: Where are you, Dr. Lecter?
Hannibal Lecter: I've no plans to call on you, Clarice. The world is more interesting with you in it.

Literature

Jeane-Claude of the Anita Blake series says he doesn't turn people often because he doesn't like the idea of taking a great artist or mind—and though vampires don't die, he says they get a good dose of Creative Sterility.
Outbound Flight:

Mitth'raw'nuruodo, brilliant tactician. Equally brilliant strategist. A being who could take on Republic warships, nomadic pirates, and even Jedi, and win against them all. And Doriana was actually considering killing him?
Doriana: *puts blaster down* Don't be absurd, Vicelord. I would sooner shatter a thousand-year-old crystal as kill a being such as this.
Thrawn: So I was indeed right about you.

This happens in Salamander, where Ellen is explaining why she hasn't killed Coelus.
In Gillian Bradshaw's The Sand-Reckoner, Hieron of Syracuse says this about Archimedes.
The Ace of Labyrinths of Echo series by Max Frei was convinced that the life of people who show a talent for the magic using forces of the Universe—as opposed to forces of the world, which anyone can in a magical enough place—is "almost sacred". Which is why he devised a complicated trap to capture the Big Bad instead of killing him. That's the man who slaughtered so much powerful mages that he became the most feared and hated being for the magical Orders in the middle of a civil war despite pretending he's no one's enemy and kills just for money (they still hired him, of course).

Live Action Television

Doctor Who: In The Seeds of Death, The Doctor invokes this when captured by Ice Warriors.

You can't kill me! Your leader will be very angry if you kill me! I'm a genius!

Real Life

Operation Paperclip was a secret mission by OSS (CIA's predecessor) to extract German rocket scientists and medical doctors after World War II to the US, regardless of their past affiliations of with the Nazi party and even war crimes.
The Greek mathematician Archimedes was supposedly killed by a Roman soldier despite orders that he not be harmed.