Killabilly, AKA "The Zombie of Zombies" is the final boss of Lollipop Chainsaw and is presumably the deity of Rotten World. Little is known about Killabilly's past life or his origins prior to the game. Killabilly was summoned by Swan, when he aspired for vengeance for the constant mistreatment he face at San Romero High.
Swan enacted a sacred ritual that would summon and sacrifice five pre-selected zombies in order awaken the great zombie, Killabilly.
It is unknown whether or not Swan actually had control over him.
Killabilly makes his debut in Lollipop Chainsaw right after the defeat of Lewis Legend. Swan reveals to Juliet that her victories against the Dark Purveyors were a pre-determined plan of his diabolical plot. He claims that the sacrifice of five pre-selected zombies was required to occur, and that by the point of their deaths they must exclaim:
Meus Vita, Rege, pro nefario coepto!
— My Life. King. for your dark enterprise!
After a flashback of all the sacrifices that Juliet had made, she questions the intentions of Swan. He claims that Juliet's "betrayal" was the main motivation of his revenge, and that he wanted to make Earth a much more "rotten" world by filling it with zombies. Swan then shoots himself with Lewis' guitar so that he may become part of Killabilly. A blob of corpses clump together at the center of San Romero, and forumulate as a giant zombie called Killabilly.
Juliet and Nick race towards him as he begins to wreak havoc. Once the two reach him, Killabilly takes hold of Juliet and a boss fight begins. After a trying battle, Juliet manages to weaken Killabilly and jumps onto his right arm. Juliet then recieves a message from Sensei Morikawa stating that she must enter Killabilly's body in order to defeat him.
As she goes to do what was instructed of her, Juliet's father Gideon suddenly appears on a motorcycle filled with grenades. He launches himself towards the face of Killabilly, and a hole is blasted through his head. Juliet mourns her father's apparent death, but is encouraged by Nick to continue, and dives down the hole into Killabilly's body. Once inside, Juliet must fight past strange tentacles coming out of the walls of Killabilly's body. Juliet then reaches Killabilly's "core" and discovers Swan's corpse attached to his heart.
Juliet is called by Morikawa again, who explains that Killabilly can only be defeated by detonating a "Nick Bomb", which involves placing Nick's head onto Swan's body. This would force Killabilly to self-destruct, killing Nick in the process. After Juliet and Nick make their final goodbyes, Killabilly dies along with Nick.
Killabilly is a gigantic zombie. His look is heavily influenced by that of Elvis Presley, with his pompadour-like hairstyle, archaic outfit and irregular morbid obesity. Killabilly has black hair that is elongated into a long curl. His body is covered in excessive body hair, prominently around his arms, chest and around his face. He is extremely obese, with several amounts of fat sagging on his stomach. He also shows some signs of significant aging, as there a few noticeable wrinkles around his face. He wears golden chains around his neck, two that happen to have pendulums with a skull logo and a diamond-like crystal with a few gold rings around his wrists. Blood is also apparent around his body, presumably coming from the corpses that composed him. His outfit is a white suit with golden trimming. The v-neck of the outfit is extended to expose his chest, creating a more "U" shaped hole. He also wears a large collar with black tap shoes and a cape.
Killabilly appears to be quite slow-witted, confused, child-like and has an irrational will to express his love and obsessions, despite being a powerful being. He is not nearly as refined or intelligent as the Dark Purveyors, as much of his speech is uttered in simple phrases. His huge size and immense power seems to trump this apparent weakness. He also seems to have a hostile infatuation for Juliet, though this and his knowledge of her may be an extension of Swan.
Guitar Swinging: Killabilly jumps back from the area that Juliet is forced to fight him in and starts swinging his arm like one would with a guitar, creating a strong wind that causes zombies and various other objects to either appear or attack Juliet.
Car Slinging: These are attacks made by Killabilly when Juliet tries to reach him. They are primarily used for lining up cars to create trails to further reach Killabilly. The player must press the Y/Triangle to cut the car in half. Each car is used to create bridges over chasms between Juliet and Killabilly.
Zombie Spit: During phase one, Killabilly frequently spits out red zombies aimed at Juliet. They can be evaded by cutting the sides of Killabilly's hand, forcing him to move his arm.
Laser Vision: Killabilly is capable of shooting lasers out of his eyes.
Gene Simmon's Tongue: Killabilly sticks out his large, long tongue in order to stun Juliet. It may also move depending on the position the player is in.
Finger Flicking: Killabilly often reache a finger towards Juliet during phase one. The player can counter by pressing B/Circle.
Toss n' Swallow: Once his health has been depleted in phase one, Killabilly tosses Juilet above his mouth and attempts to swallow her. The player must mash B/Circle repeatedly in order to survive. If the player passes, Juliet jumps into his mouth and cuts the inside of it, causing Killabilly to spit her out. If the player fails, Killabilly chews Juliet's mid-section and swallows her along with Nick.
Claw Clap: Killabily strikes at Juliet with his massive nails.
Big Boy Truck: Killabilly takes a car (primarily a white truck) that chases Juliet and explodes after a certain amount of time.
HelicopterSmash: At random points, usually after the use of his Laser Vision, a helicopter suddenly appears on stage. Killabilly smashes it out of irritation, and the player must either avoid or destroy it.
Poor Executions: Despite Killabilly's massive strength, his low intelligence prevents him from properly destroying Juliet. This gives her an easy chance to defeat him, as his own innocence led to his demise.
Killabilly appears to embody what Swan's own malicious plans had made him to become. He may symbolize how Swan's plans have negative consequences for him in the end. Killabilly is a large monster who shows an obvious sign of innocence. This could symbolize how Swan's revenge has caused him to become a "monster" himself, but was really just a victim who was led by emotional sorrow. Killabilly's death could also symbolize how Swan felt when Juliet chose Nick over him, as Juliet "broke" his heart by showing her love for Nick and not Swan.
His age is the Number of The Beast ("six hundred threescore and six") as prophesied in the Book of Revelations (Revelation 13:16-18).
The boss fight against him pays homage to the Stay Puft Marshmallow Man from the Ghostbusters films.
The achievement I Came, I Saw, I Kicked Its Ass is probably also a reference to the first Ghostbusters film, in which Dr. Peter Venkman says: "We came, we saw, we kicked its ass," after trapping the green slime ghost in the hotel.
Both are a parody of Julius Caeser's "Veni, Vedi, Veci." (I came, I saw, I conquered).
He has a passing resemblance to the Cyber-King robot from the Doctor Who TV series, as Killabilly and the Cyber-King are both gigantic entities which require control by a human host, as the Cyber-King is controlled by Miss Hartigan. Killabilly and the Cyber-King both also were created for absolute destruction of humanity.
His large tongue may be a reference to Gene Simmons, of the American Rock Band KISS.
The entire boss battle is very similar to that of the final boss in the 1997 Playstation platformer Klonoa: Door to Phantomile, and its 2009 Wii remake. The final boss of Door to Phantomile involved the protagonist, an odd canine/feline hybrid named Klonoa, fighting a gigantic, disgusting beast resembling a slug or amphibian. During the first phase, the boss would shoot enemies out of a hole in its back (which Klonoa had to grab and throw at his friends' cannons in order to blast the monster), and try to catch Klonoa with its long tongue. After completing the first phase, the beast eats everything in its path, so the next two phases take place inside its body, whose body seems to be another world. In the third phase, Klonoa's best friend, Huepow, tells him that in order to defeat the inner monster and the monster itself, he must use Huepow as a weapon and sacrifice him. Afterwards, when it is believed that Huepow is dead, he returns in a new form (his true, human form).