Likewise, the Elrics end up needing their former Anti-Hero enemy Scar in the manga for his literacy skills in order to decode his deceased brother's counterplan to the villain's master plan.
Kimimaro of Naruto stated that he fully intended to kill Tayuya once they returned to Orochimaru. The only reason he didn't do so immediately was that she could still be useful in slowing down the Leaf ninja.
Invoked in One Piece by Tom. His trial was an open and shut case and he was sentenced to death. At which point Tom reveals that he can build a new form of travel that will revitalize the Wretched Hive they live in, earning a stay of execution.
The Hunchback of Notre Dame, though the main reason Frollo can't kill Quasimodo as a baby is that the Archdeacon caught him trying and guilt-tripped him out of it. He does, however, state that he's going to find a use for the "foul creature."
Curse of the Black Pearl: Jack and Will steal a ship, then fight when Jack tells Will his father was a pirate. Jack gets Will down with a sword to his neck, then reverses the sword, admitting he needs Will to sail the ship.
Jack Sparrow: "And me, for example, I can let you drown, but I can't bring this ship into Tortuga all by me onesies, savvy? So, can you sail under the command of a pirate, or can you not?"
Later in the film, Elizabeth burns all the food, the shade, and the rum to signal a passing Royal Navy ship. Jack, who normally is reluctant to kill (which makes him a somewhat ineffective pirate in most areas), is so angered by this that he pulls out his pistol in a rage to shoot her in a moment of Black Comedy, but he stops himself when he realizes that in order to reclaim the Black Pearl and rescue Will, he will have to smooth-talk his way onto another ship. Which, thanks to Elizabeth, will be a Royal Navy ship. Shooting Elizabeth would remove the only thing keeping them (or Will) from executing him on sight, as well as waste the one shot he's been saving for Barbossa.
At World's End: Jack tricks Beckett into thinking this by telling him he can convince the other pirates to meet Beckett's armada head on rather than holing up in Shipwreck Cove.
In Street Fighter, this is why M. Bison allows Dhalsim to live after Dhalsim makes the mistake of pressing Bison's Berserk Button by calling him psychotic.
In Abandon Ship there are too many survivors for the one lifeboat. Most of the people are left clinging to the outside of the boat but the captain allows one passenger to pull his huge dog in. The dog's owner is incredibly grateful but another survivor demands to know why the dog gets in while he doesn't. The captain's answer was "If we're not picked up immediately we could be out here for a long time. If that happens, we can't eat you." Cue the horrified look on the dog owner's face.
In Crimson Tide, a US submarine gets word that some Renegade Russians are preparing to launch some nukes they've managed to get ahold of and are ordered to launch their own nukes first. Then a second message is Lost In Transmission regarding the launch, and the crew soon tears itself in two between those who want to launch nukes now, and those who want to confirm the intent of the second message. Eventually, the Captain is put in a position where he is threatening another officer with a gun to try to and get the launch codes. The officer, who is the only one who can access those codes, refuses to turn them over. The captain counts down... and then doesn't go through with the shooting it because if he kills the officer, no one on the ship will be able to use the nukes. The Captain gets around it by threatening to shoot the officer's subordinate and friend instead.
In Star Trek Into Darkness, Spock is inches away from killing Harrison when Uhura stops him, because they need Harrison's blood to save Kirk's life.
Ma-Ma in Dredd doesn't kill one of her lieutenants for getting her drug operation exposed, but only out of practical concerns, because Dredd has already killed too many of her soldiers by that point and she's short on manpower.
In Diamonds Are Forever, Shady Tree rescues James Bond from Albert Wint and Charles Kidd's incineration attempt on him because he had passed phony diamonds on to Shady, and Shady wants the real stuff. Albert R. Saxby tries to rescue Shady Tree in the same manner, but as James soon finds out, Saxby had talked the two assassins down too late and Shady had already been shot in the head by Mr. Wint.
In Lockout, Alex kills one prison engineer, the second one has a problem getting the door open, so Alex has a third engineer brought up, shoots the second, hands the third the door control the second was trying to activate, and tells him he has two minutes to get the door open. Realizing this isn't enough time, the engineer tells him, "You can shoot me, but I can guarantee I can get the door open in five minutes." Alex is pleased, admitting how he liked that this third engineer knows how to negotiate, and lets the engineer live after he does what he said he would.
Happens more than once in Duumvirate. The biggest example is when William wants revenge on his creator for enslaving him to his own brother, but this trope kicks in instead.
In the Warrior Cats series, the Big Bad, Tigerstar, decides to spare Featherpaw and Stormpaw because he thinks they may still be useful to him. (Presumably as hostages, since they're the children of his enemy's best friend.)
This is Mara Jade's opinion of Luke Skywalker in The Thrawn Trilogy. She wants revenge for the death of her old master, the Emperor, but always finds herself relying on him for survival or some other more immediate problem. As she comes to realize, it's actually not her desire for revenge; it's the last command of the Emperor, given to her as he died—not that this makes it any easier to ignore; if anything, her resistance to the command actually makes the compulsion stronger. She eventually frees herself from the compulsion by killing Luke's Evil Clone instead.
In the second book of the Chaos Walking trilogy, the protagonists finally have the Big Bad at their mercy. Then, the native species of the planet launch a war against all humans, and they realize that they need him to command his army in order to survive.
In John Birmingham's Final Impact, the "uptimer" Australian submarine Havoc wipes out the remains of the Japanese fleet, except the carrier of the the "Ohka" kamikaze jets. Just before the battleship Yamato is destroyed, Admiral Yamamoto receives a secret message from the "uptimers" with the location of the Soviet nuclear development facility in Kamchatka. Yamamoto sends the order to the Ohkas just before the Yamato is hit by a torpedo. The carrier launches its suicide fighters, who wipe out the Soviet facility. The Havoc then finishes the job by torpedoing the carrier in order to remove any witnesses.
Moriarty seems to fully intend to kill both Sherlock and John during the swimming pool confrontation, but backs off immediately after he gets a phone call. Apparently he still needed Sherlock alive so that Sherlock would solve Irene Adler's code for him.
In season 4, Uriel tells Sam to cease his demon blood consumption at once, because the minute he stops becoming useful to the angels, he intends to kill him.
In season 5, the boys and demon Crowley capture another demon who serves and looks after the Four Horsemen. They need him to reveal the locations of Pestilence and Death, but the situation is complicated because this demon is revealed to have tortured and roasted Jessica, Sam's deceased love interest. The demon gives a disgusting gloat about his crimes, and Sam can just barely resist killing him on the spot and getting his revenge because they still need him.
In the seventh season, a slipping Castiel does this to Crowley because he still has use for him despite Crowley's counterbetrayal to him the previous season. He directs Crowley to downsize Hell and maintain the Cage for the two remaining Archangels. Crowley happily obliges.
At the end of season 8, Dean and Sam capture Crowley for the last trial. In season 9, they still have him locked in the basement, citing this trope, though he hasn't been useful to them yet.
Adria: I extended my protective shield. (looking at Daniel)You're lucky I still need you.
In Game of Thrones, Ramsay Snow claims this is why he won't kill Theon Greyjoy. It's pretty obvious that the real reason is that he's having too much fun torturing his prisoner.
In Dino Attack RPG, this was the main reason Baron Typhonus stepped in with his Divine Intervention to save Dr. Rex from death. He would not so easily let his strongest tether to LEGO Planet simply die, even if that meant keeping said tether in a perpetual Fate Worse than Death.
Stand Up Comedy
John Pinette's routine of his visit to France, wherein he learned how close he was to the French-Italian border (and Italian food). After spending several weeks in France.
I'll kill you later 'cause I need directions.
In World of Warcraft, many quests that directly involve the Lich King will have him letting you walk out alive for seemingly no reason; in one such quest exclusive to Alliance characters, he will even overtly say It Is Not Your Time. Turns out this trope is the reason; he's actually letting you kill his most powerful troops to test you and make you stronger so that when you reach the Frozen Throne, he can kill you with just a snap of his fingers and raise you under his service. This is exactly what happens when you fight him. You're only saved by a last minute Deus ex Machina courtesy of Tirion Fordring and the Ashbringer.
Dragon Age: Origins: If you already know the final Reveal (that one of the Grey Wardens in your party must perform a Heroic Sacrifice after the Final Battle), this may become a motivation to spare the life of Teyrn Loghain and instead recruit him to the Grey Wardens. Riordan tries to argue in favor of doing so without publicly stating why.
This is the standard Paragon outcome to Zaeed's quest in Mass Effect 2. If you've already completed the final mission at this point, however, you can leave him to die instead.
Similarly, in Mass Effect 3, Shepard can choose to spare Balaknote Or, if Balak was killed in the first game, another batarian terrorist. in order to get the batarian fleets on her/his side. It's made exceptionally clear that Shepard would love nothing more than to kill him on the spot, but there's too much at stake to murder someone who could bring extra firepower to the war against the Reapers.
Officer Noles: Is that... Do you want me to arrest him?
Shepard: I want you to put a bullet in his head, but we're all making some sacrifices today.
Fable III: If you choose to spare Logan in the third game, the Hero Prince explains that "now is not the time for revenge.", politely asking for his brother's help. A kind way of putting it- you spare Logan because you need his built-up military to assist you.
Dominic Deegan includes a couple of these. Notably, Evil Necromancer older brother Jacob including 'don't kill Luna' as a major part of his demands when negotiating with the Shindula, simply because he expects to get some use out of her... for now.
Of course, he has his Heel Realization and saves her just 'cause at the end of the arc. "Don't say I never did anything for this family."
In the final episode of the first season of Young Justice, Vandal Savage notes this about the Justice League, explaining that they are still needed for Phase Two of the plan, mind-controlled or not.
In the first serial of Rocky and Bullwinkle "Jet Fuel Formula" Boris keeps getting contradictory orders from Fearless Leader - keel moose, or don't keel moose - depending on whether or not they think they have all the ingredients for the rocket fuel Bullwinkle accidentally cooked up.
In the five part pilot for Ducktales, Glomgold and the old captain attempt to kill Scrooge and his nephews to keep them from reaching the lost ship first. Then they discover their map being eaten by their donkey and hastily undo their attempt on Scrooge's life.