Saizo: We should just kill them all… One day we will. When that day comes, will you remember which side you're on?
Tsukune: I'll never forget that.
He Who Fights Monsters is a Rosario+Vampire fanfic written by Hawker 748The story is about what would happen to Tsukune if Moka hadn't crashed into him on the first day of Youkai Academy. While other fanfics generally try and keep the fanservice, comedy and flirting, this one is based around the horror side of the series, asking exactly how far someone would go to survive in an environment where the slightest flaw means weakness, followed shortly thereafter by painful death.
Ascended Extra: Keito was a minor villain who only appeared for one episode in the original series. Here her role has increased to main character and Tsukune's girlfriend.
Asshole Victim: While Tsukune has quickly decided that turning to murdering anyone he fights is the best way to solve his problems, with the debatable exception of Inner Moka, pretty much everyone deserves what's coming to them.
Ax-Crazy: Tsukune becomes this and acknowledges when speaking to Headmaster.
Breaking Speech: Tsukune uses this on on the Headmaster by calling him out on his neglect and the stupidity of his plan for human-monster peace.
Bring My Brown Pants: Tsukune accidentally makes a girl piss herself in fear when he almost kills her boyfriend, Hikaru, who is also his old bully.
Bullying a Dragon: Tsukune's third fight opens up with what is essentially this. Though Tsukune is just a normal human, he's made it clear to the school population that he's more than willing to ruthlessly take someone out if he needs to do so. The person bullying him is aware of this.
Chick Magnet: To Tsukune's confusion/dismay he gets more fangirls each time he kills someone.
Death by Adaptation: Inner Moka. And the last we see of Kurumu is her being taken to Keito's office for "punishment".
Not exactly the last thing so far. Tsukune finds Kurumu's hairband and bra in Keito's Enforcer office. It's all but implied, yet it makes one wonder what really happened to Kurumu there.
Ruby has recently joined them.
Death by Childbirth: Keito claims she never knew her father, and her mother died soon after giving birth to her. Now consider she's a Jorōgumo. Anyone who knows the basics of some spider reproduction habits should realize this strongly implies that Keito's mother ate her father, and then Keito devoured her mother soon after hatching. It's no wonder she advises against she and Tsukune having children.
Deconstruction Fic: The story is about what Tsukune would be like if he didn't have Moka to protect him and what an ordinary human like him would have to do to survive in a place were everyone is much bigger, stronger and faster than he is.
Defiant to the End: Tsukune, when he thinks he's about to die. Specifically, trapped and with his humanity revealed, Tsukune's grimly refuses to beg for his life when given the opportunity and decides to focus on working up enough saliva to spit in Keito's eyes before he dies.
Demoted to Extra: Mizore's only appearance as of chapter 11 was a brief glare at Tsukune after his first murder. In fact, most of the main cast has gotten this treatment. The ones still alive that is.
Hope Spot: In Chapter Eight, Moka approaches Tsukune, and is heavily implied to have been working up the courage to say that she likes him. There's even accidental groping, and the story has a moment where it just might turn out alright. They don't. Oh, God, they don't.
He Who Fights Monsters: The whole basis of the plot is Tsukune slowly becoming this trope. Heck, it's the name of the story.
Hero Killer: Tsukune is an Inverted version of this trope taken Up to Eleven. He's the main protagonist but fits all of the qualifications from being The Dreaded, to having a high bodycount and killing a main character.
Heroic BSOD: Tsukune has a breakdown after killing a fellow student for the first time.
Humans Are Warriors: Tsukune has quickly proven himself to be the most lethal student on campus so far, deadlier than even Inner Moka, who he kills. Tsukune even points out to his fellow students, after having killed two monsters, that humans are experts at fighting dirty.
I Choose to Stay: Tsukune is finally given his transfer papers but immediately rips them up, telling the Headmaster that he's not getting rid of Tsukune that easily and that he no longer belongs amoung humans. He also does it to spite the Headmaster.
As of the final chapter, Tsukune has become a teacher at Yokai Academy.
Improbable Weapon User: Tsukune has used hairspray and a lighter, a sharpened butterknife, a fire hose, a chair leg, and several bags of salt to kill six students, including Inner Moka (not all at the same time; the challengers come one at a time), while defending himself.
It Gets Easier: Tsukune is initially feels guilty and disgusted when he is forced to kill an opponent. However, by the time he makes his fifth kill, he's barely able to restrain himself from murdering Yukari in cold blood when she gets angry at him for killing Moka.
This gem from chapter 11 highlights it nicely.
Tsukune: "When I have a problem with someone, my first thought isn't how to reason it out, it's about where to find the sharpest object and how much force it would take to get it through a ribcage!"
Jumping Off the Slippery Slope: Tsukune first only kills when he has no other choice. In the final chapter he preempts an attack kills the one plotting against him ahead of time
Meaningful Name: A slightly meta example. Chapter Eight is titled 'Rubicon,' and the phrase 'Crossing the Rubicon' is used to describe passing a point of no return. Chapter Eight is when Tsukune kills Moka.
Memetic Badass: In-universe example. Over the course of the story, people come to believe Tsukune can and will kill anyone who even slightly annoys him. A student even asks if he really killed two vampires after jumping out a 10th story window.
Moral Dissonance: Tsukune can't believe that others start respecting him more after he kills a fellow student.
Mugging the Monster: Bullying the hardened Tsukune was the biggest (and almost last) mistake of Hikaru's (Tsukune's old bully) life.
Murder Is the Best Solution: This is Tsukune's default thought process. Everytime he meets someone his first thoughts is how will he most effciently kill him or her if it comes down to a fight. This is why he chooses to stay at Youkai Academy.
My God, What Have I Done?: In the final chapter, Kasumi Aono breaks down after learning just what Tsukune has gone through, and that he endured all that hell and has been irrevocably changed because he didn't want to disappoint her.
My Greatest Failure: Tsukune is hellbent on becoming this for the Headmaster just out of spite.
Mythology Gag: While not word for word, Hikaru tells Tsukune to "know his place."
Nice Job Breaking It, Hero: The Headmaster intended Tsukune to show the school that humans and monsters can live together in peace. His lack of interference in Tsukune's life and utter lack of protection turned Tsukune into the most feared person in the school, to the point that even Enforcers are afraid of him, and proved the assumption of "Humans Are Bastards" utterly correct.
No-Nonsense Nemesis: Tsukune is once again a heroic version of this trope. All Youkai like to grandstand, gloat about their superiority and power, and follow shonen fighting tropes. Tsukune just kills you without a word.
Not Hyperbole: Tsukune when he tells Hikaru's girlfriend that Keito would kill her for trying to seduce him.
Pre-Mortem One-Liner: Tsukune doesn't normally do this but he does give a awesome one liner to the first monster he kills
"Fire needs three things to exist: Heat, oxygen, and a source of fuel. Guess which you are!"
Relationship Upgrade: The End of chapter 8 and all of chapter 9 centers around Tsukune and Keito going through this.
Sanity Slippage: As of chapter 11, Tsukune has become aware that this is probably happening to him.
Scar Survey: Keito asks Tsukune about each of his, leading to his realizing just how many he has.
Selective Squick: Subverted after Keito uses Tsukune's toothbrush. He briefly worries she might have brushed her mandibles with it then realizes that after everything else they did the night before, it hardly matters.
Shut Up, Kirk!: The Headmaster's attempt to call Tsukune out gets turned around into this.
Stranger in a Familiar Land: Tsukune recognizes that his experiences changed him into someone who is too violent to fit into any civilised human school.
Super Window Jump: Tsukune does this when Moka keeps blocking his attempts to flee via the stairs.