"My friend is dead, Patroclus, my dearest friend of all. I loved him, and I killed him...Hector cut him down! I no longer have the will to remain alive among men, not unless Hector loses his life on the point of my spear and pays for despoiling Menoetius' son."
Killing off a character's closest friend can have serious consequences.
At this point a hero may become much more serious at his work. For a character who already rides the fence between good and evil, this can be enough to totally push them over the edge. And both will sometimes find a way to blame themselves for it. Optional routes are a nervous breakdown, and/or a brief retirement. It's likely the hero will take an I Work Alone attitude, at least for a while. In Anime this also can occur with the death of partner Mons.
- Dragon Ball Z: After Freeza (who was assumed dead) emerges out of the ocean, he wastes no time killing Goku's best friend, Krillin. Goku goes over the edge and turns into a Super Saiyan, then kicks the crap out of Freeza. Piccolo Daimaou made roughly the same mistake nine (in series) years earlier, and Super 17 would make it again 34 years later, with roughly the same results.
- Takeru's Angemon in Digimon Adventure died (and came Back From the Dead) when Takeru was a small child; the resulting trauma led him to become a warrior for justice, and about the only one in Digimon Adventure 02 that argued for killing the bad guys instead of just locking them up. This wouldn't be the only time a partner was killed -- Wormmon, Leomon, and Masaru's Agumon are the other offenders. However, Leomon didn't come back.
- Leomon's death, in particular, was taken exceedingly well.
- Not quite a sidekick, because she was rejected as such by Mai, but whenever Sayuri in Kanon was hurt by the demons (which was usually pretty badly, and she would have died were it not for Mai's healing), Mai went almost crazy in her attacks on them.
- Played with in Yu Yu Hakusho during the Dark Tournament Arc: Yusuke lies on the arena floor beaten by Toguro, who provokes Yusuke to go on fighting by attacking and killing Yusuke's best friend Kuwabara. Not only does Yusuke find the strength to stand up again, he also proceeds to thoroughly kick Toguro's butt. After the battle is over, however, Kuwabara reveals that he's Not Quite Dead. Toguro missed his vitals, but Kuwabara played dead, knowing that his "death" would give Yusuke the determination needed to defeat Toguro. Of course Kuwabara almost suffered death AGAIN when Yusuke realized he'd been fooled, and only the collapse of the arena snapped him out of the semi-murderous rage.
- Mercilessly subverted in Episode Five of Mnemosyne. Mimi's stronghold is attacked by a cyborg Laura and a squad of Angels. Mimi herself is kidnapped and Rin rushes to the top of the temple to save her, expecting the worst. However, instead of killing Mimi first, Apos cuts to the chase and just kills Rin instead.
- Played with in Tengen Toppa Gurren Lagann. The hero Kamina is killed, only his sidekick was actually the real hero and Kamina enabled him. Suffice it to say an angst-fueled breakdown ensued.
- This happens to Batman quite a lot. The death of Jason Todd, the second Robin, had a long-lasting impact on both Batman and the role of Robin, to the extent that Batman considers Jason's death the greatest mistake he'd wish to fix. The current Robin describes his entire career as being spent under Jason's shadow, and other characters define Jason's death as the turning point after which Batman became Darker and Edgier.
- Captain America (comics) had long felt guilty about his sidekick Bucky Barnes's death, occasionally falling into angsty despair over it. This was used against him by various villains from time to time. It used to be the major flaw of Modern Cap in his earliest days until Stan Lee and Jim Steranko recognized how old it was and had Rick Jones tell Cap to "Quit Your Whining" and get over it. Bucky, however, isn't quite dead...
- Played with in Earth X and sequels: most of Captain America's sidekicks (not just Bucky Barnes) have died. In Earth X, "Daredevil", whose ability to regenerate damage prevents him from dying, teams up with Cap in the hopes of being killed. It doesn't work, but Daredevil eventually gets his wish. In Universe X, Cap becomes the sidekick of the reincarnated Captain Mar-Vell, and promptly gets killed mid-series.
- In a rare occation of the sidekick being killed by the hero, in Irredeemable The Plutonian lobotomized his sidekick Samsara after his Face Heel Turn. This was assumingly done to keep him from telling people the secret to his powers.
- In the film version of The League of Extraordinary Gentlemen, the whole reason Tom Sawyer joined the League was because the Fantom had murdered Huck Finn. Note that this is not included in the actual film; it's revealed in a deleted scene shown only on the DVD.
- Boltie in Super.
- Constantine. The title character has three of his sidekicks die on him during the movie: Hennessy, Beeman and Chas Kramer. Chas finally gets a taste of the action and helps subdue Mammon only to be killed by Gabriel midway through his bad ass favorite phrase.
- After Achilles retires to his tent due to a quarrel with Agamemnon, his sidekick Patroclus dons his armor and rallies the hard-pressed Achaeans, only to be killed by Hector. Achilles hears about this and snaps, going on a Roaring Rampage of Revenge.
- Older Than Dirt: In The Epic of Gilgamesh, the story turns from being a story about the shenanigans of two super-powerful best buddies to a tragic tale of the futile quest for immortality on the death of Gilgamesh's Enkidu.
- In The Hunger Games Katniss's ally, Rue is killed. This is the first thing that really helps set Katniss off against the Capitol, as she sees Rue's death as their fault because they're the ones who made the Hunger Games in the first place.
- The Future Badass version of Hiro in Heroes went from idealist to Zen Survivor due to the death of Ando.
- On Hercules: The Legendary Journeys, Herc's li'l buddy Iolaus is possessed by an evil god and killed. And replaced by his Mirror Universe comic relief doppelganger (nevermind the previously-established rules that said if you die, your Mirror Universe counterpart also dies). And eventually brought back in time for the finale.
- Happens to the Doctor from Doctor Who on occasion, to the point where Wikipedia has taken note.
- The Doctor's companions very rarely die though. They may leave voluntarily, be returned home and/or mindwiped or be stranded in a parallel universe, but the vast majority survive their travels. The only long-term companion to die while traveling with the Doctor is Adric, and it's clear the Doctor feels somewhat responsible for his death. Indeed, the Fifth Doctor's last thought as he regenerates is "Adric?"
- Hawk's death at the hands of the Big Bad in Soldier of Fortune.
- The World Ends With You Rhyme to Beat. Of course Beat, Shiki, and Joshua themselves count as dead sidekicks.
- Fairly common in RPGs is to have a cast member who's important, but not too important to the main character or plot die to heighten the tension. Final Fantasy for example has Galuf in V, Aerith in VII, possibly Shadow in VI. Mass Effect can have Wrex on Virmire if your Charm or Intimidate aren't high enough. Also, you have to choose whether to save Kaidan or Ashley die there. The person you don't pick dies and you and the remaining character will grieve for them.
- Toyed with in, of all things, Pokémon Black and White. The loss of one of his Pokemon to disease was the big reason Alder left the Unova League to its Elite Four to wander in mourning. He manages to come around as a result of Team Plasma's heightened activity, especially when N gets ahold of Reshiram or Zekrom. If Ghetsis was counting on using this knowledge in an attempt to demotivate Alder, it certainly didn't work.