Any time Shiki's Superpowered Evil Side, Nanaya, makes an appearance. Without fail, something will die, and in a very awesome way. The manga seems particularly fond of this.
At one point, he gets pissed at some zombies pinning down Arc because she's his woman—if anyone's going to molest her, it's going to be him. *slice and dice*
Shiki's fight with Nero, an already noteworthy event from the Visual Novel made even more awesome by the emphasis on Nanaya-mode. Collapsed on the ground and surrounded by demonic familiars about to devour him, he snaps, shredding everything around him and laughing crazily as their blood rains down upon him.
Nanaya Shiki: "You wanna kill me, monster? Very well. Come on, then, let's kill each other... Nero Chaos."
And, of course, Shiki's climactic battle with Roa in the school. Shiki gives a monologue that terrifies Roa, then takes him by surprise by killing the hallway around them. This quote sums it up very well:
Shiki: "Life and death will be back to back, but they'll never face each other. [...] I'll show you... THIS is what it means to kill something." *cue ass-kicking*
Also: Shiki: "....You're not scared are you? It's a path you're quite familiar with. If there's a difference, it's only one thing. —-This time, you can't come back."
Of course, the manga makes this even scarier for Roa and turns Shiki into a SNK Boss, who kills every single magic attack from Roa! Also, after he destroyes the hallway, he doesn't use the stairway to follow Roa like in the Visual Novel or the Anime... HE JUMPS AFTER HIM AND KILLS ROA IN FREE FALL!
And let's not forget to give props to Roa as well in that scene, after getting dot stabbed by Shiki and having his legs crushed by debris he's STILL alive. Roa then crawls over to Shiki, who is nearly comatose due to both his physical stunts and overuse of his eyes, and attempts to pull a Grand Theft Me on Shiki. If it weren't for Ciel getting the drop on Roa there's a good chance he would have succeeded.
The fight between Akiha and Ciel in the manga certainly qualifies, both because of how savage we see Akiha get whenever her Onii-san is threatened, but also because late in the fight after Ciel reveals she's immortal, she takes a point blank shot from Akiha that burns her hand down to the bone and regenerates it, all without flinching. Akiha can just stare on in disbelief as it happens.
Let's not forget how Akiha wtfpwned SHIKI in the end of Hisui route. If only she had those mad killing skills ready in her own route... ;_;
You don't call Vermillion Akiha ripping SHIKI apart to be mad killing skills?
The manga keeps topping itself. That fight is still a crowning moment for Akiha. Ciel gets a new one when she fights Roa...including at one point pulling a Gate of Babylonstyle attack using Black Keys.
The Prelude side story begins with Satsujinki—a future Shiki whose eyes have grown too strong for his glasses to contain, thus forcing him to cover them in bandages—killing a DAA so silently and efficiently that he doesn't even realize he's dead until it's too late. This is made all the better by the fact that a group of mages had been preparing for months on end to attack the DAA, but Satsujinki just literally walked in overnight and killed him undetected.
Made even better by the group of other mages including Lorelei Barthomeloi, who is the single most powerful mage still completely human and capable of fighting on the same level of the Dead Apostle Ancestors and winning alone, ed Enhance, the Dead Apostle Ancestor who hunts down other Dead Apostle Ancestors and their clans for a living. And yet he walked in, pulled an Enhance (as in killing both the Ancestor and every other Dead Apostle inside) and was not noticed until he stumbled on them by accident.
He's also in the Talk side story (uploaded to link), where he (and Ciel, they met up by coincidence) kills a living forest Dead Apostle (off-screen) to get the blood-fruit in the middle for Arcueid, whose hunger is starting to overpower her. While in the forest, he forces a mage/swordswoman who can go toe-to-toe with the Burial Agency to run away just by looking at her.
Satsujinki is a Meaningful Name - it roughly means "bloodthirsty murderer," but the last kanji is replaced with one from Shiki's name, making it roughly translate to "Shikiller." Satsujinki sounds better, though...
There's a doujin where Shiki sees a star where nobody else sees one, and then realizes that's the Killing Spot for the universe. He makes a note of it, in case in the future the universe "requires to end." Think about that for a second.
The limits of his ability have been argued to ridiculous levels at Beast's Lair. General consensus is that, while he wouldn't be able to safely kill anything outside of or greater than Gaia's cycle (non-Gaia beings = vastly different concepts of death), he is still the ultimate hax.
General consensus nothing, that's part of things stated by Word of God about Shiki not being able to kill thing's like ORT because it's not part of gaia and thus immune to the gaian order of death. And even within Gaia, there are things that are immune to his eyes ala Arcuied under a full moon or Tatari in phenomenon form.
Killing the universe is explicitly impossible for Shiki Tohno. It is, however, perfectly possible for Shiki Ryougi.
Kagetsu Tohya also provides belated characterization of SHIKI, who wasn't portrayed as sympathetic in the original game until the very end of Hisui's route or in a dream sequence during Kohaku's. In "Drinking, Dreaming Moon", Shiki and SHIKI are able to talk on friendly terms, and finally cry the tears they should have shed when they killed each other in a Crowning Moment of Heartwarming and Tear Jerker.
That wasn't a dream sequence...That is the only route where Shiki really IS a killer for a time...
It is, he's referring to Kohaku's route, in which Shiki talks down SHIKI by telling him hes a much better killer in what appears to be a dream...until he wakes up in the morning with blood on his hands.
The fight against the Tatari with the appearance of poor Satsuki in the manga of "Melty Blood": Especially the point, where Tatari!Satsuki managed to pin Shiki down and punched the cr... his blood out of him! ...Until it realized that Shiki wasn't under it anymore and the blood on its hand didn't belong to Shiki... And then its arm already fell off...
It has to be said. As bad as the anime adaptation was, and as out-of-character as it would be for Shiki to sit there and take it, Arcueid, Ciel, and Akiha getting into a verbal catfight over him makes perfect sense...but it's the fact that Satsuki finally jumps out of her seat and tells them all to shut up, sit down, and leave Shiki the hell alone (which they do, possibly out of shock) that feels oddly satisfying. Go non-vampire Sacchin!
The most awesome thing Roa ever does is in the Ciel Good Ending, where Shiki (who has almost been entirely taken over by Roa) dot-stabs himself in a last-ditch effort to kill Roa (two souls fighting over one body means that his ability can kill off either of them, leaving his body for the remaining one). He collapses and has a vision of an unknown man reading a book. When Shiki asks him if the book is interesting, the man smiles and tells him that nothing was interesting at all, then gets up and walks away. Shiki tells him he forgot his book and the man replies that he doesn't need it anymore, so Shiki asks where he is going and the man responds that he was never anywhere to begin with, and fades away as the book's pages scatter and disappear. The scene is so awesome, it changes your entire perception on Roa that you had throughout the game. It really is a quite a profound scene.
The book carrying the memories of long reincarnation disappears. And the vampire called Roa dies gracefully after eight hundred years.
Melty Blood. Especially some of the endings.
Ryougi facing a full-power Archetype: Earth at the end of her story mode.
Ryougi: Hmph, don't regret saying that vampire, or whatever the hell you are. It doesn't matter how many tens of thousands or hundreds of millions of lives you have. Either way, this is the end of the dream. Let's do this- As long as it's alive, I'll even kill gods.
Roa killing Arc causing Gaia to activate a fail-safe trigger, which brings back the original 100% Arcuied from all those centuries ago.
Roa: Ha... this is it! This is what I wanted! 800 years ago, I HAD seen eternity! The princess of the moon, not fading even a bit since that moment! I present to you to the spectacle of the burning end of my 18th death, my final reincarnation!
Nanaya avenging his family by killing Kouma, one of the few people Nasu stated to be able to fight against Heroic Spirits (even though he does die in the process)
Nanaya: Oh, my bad. I tore open your throat so you can't talk now. Ah fuck, I can't see now. I've got too much blood pouring out now.
Hisui, surprisingly, and appropriately, gets one in her Good Ending. When she and Shiki go to try and help Akiha and Kohaku fight SHIKI, Kohaku takes advantage of the confusion to make herself a target, forcing Akiha to take a (hopefully fatal) blow to save her. This being the good ending, Akiha survives, and when Hisui recognizes what Kohaku was trying to do she finally loses it, telling Kohaku to quit fucking around and tend to Akiha's wounds while Shiki finishes SHIKI off. Then she gets a second one when she calls bullshit on Kohaku's Motive Rant to Shiki afterwards, both for attempting to get Akiha, the closest thing Kohaku has to a best friend, killed, and for claiming to hate Shiki while using the personality she took from Hisui all those years ago, since she could never hate Shiki. Her usual stoic character and being the only heroine to lack awesome combat powers or manipulating skills makes it all the more awesome to see her standing up to protect the people she cares about.
I agree with the awesomeness of the scene, but at this point Kohaku's speech is less a Motive Rant and more Kohaku trying to justify why she became the Spanner in the Works for her own Evil Plan, within the context of her 'doll' delusion.
Shiki and Ciel's Big Damn Heroes moment in the Alliance of Illusionary Eyes side story. Granted, Ciel's entirely offscreen, but just when the Fake Shiki's savoring his newest kill, pressing his knife to Seo Akira's chest, Ciel manages to knock his hand away with one of her Black Keys. Think about that: she just pulled off Blasting It out of Their Hands with throwing swords and managed not to harm the innocent. Then Shiki makes his presence known, sets the record straight on who's who, delivers a lecture on the nature of life and death rivaling the "World of Cardboard" Speech given to Roa, and then knocks out the impostor in one hit. To top it all off, Seo now has a huge crush on him after this, the first time they've actually met.
Nrvnqsr beating the shit out of Shiki without noticing OR turning to face him. While he was using a majority of his power to trap Arcuied in the Soil of Genesis.
Shiki instinctively turning into assassin-mode near the beginning of the game upon first encountering Arc, stalking her back to her apartment and then slicing her into 17 pieces in a perfectly-executed surprise attack. (She gets better.)
Shiki fighting a berserker Arc at the end of Ciel's route by killing a part of the Earth itself to disconnect Arc from her power.
Kohaku, at the end of the Hisui arc when it turns out that the happenings of pretty much the entire game were all part of her big plan to kill off the entire Tohno family. And then commits suicide right in front of the man she's secretly been in love with for years.
Ciel's reaction to trying to understand what Shiki can do. "What kind of an artifact is this knife?" TO put it another way, the idea that Shiki is capable of doing these things, is beyond Ciel's comprehension. And Ciel has all of Roa's knowledge...