Genocide Route

The Genocide Route, also known as the Kill Everything Route, the No Mercy Route, or the  Bad Time Route, makes up one of the three main possible endings to be found in Undertale. It can be achieved in any playthrough with no pre-requisites, although any subsequent Pacifist Routes will have their Epilogue slightly altered, even with a True Reset.

Method
The Genocide Route is achieved by killing every enemy in every region - The Ruins, Snowdin Forest, Waterfall, and Hotland/Core (For the Genocide Route, Hotland and Core share the same enemy counter).

As a note, this does not mean that the route is achieved by killing every enemy that the player happens to encounter. Instead, the player needs to kill enemies in encounters until encounters with enemies no longer occur in each area. After killing all the monsters, encounters will appear, but they will have no enemies and the text 'But nobody came.' Upon seeing this message, the music for the area will be changed to a distorted-sounding ambient track, instead of the regular music both in and out of battle.

The number of remaining enemies is displayed at save points starting in Snowdin. The Ruins does not provide an enemy count at save points, however, their save messages will still change to "Determination." when all monsters in the area have been killed.

Additionally, to satisfy a all bosses must be killed.

If the player doesn't meet the requirements for a then they will enter the Neutral Route, where the game's normal soundtrack plays and more NPCs can be found.

Requirements

 * Exhaust the monster kill counter in every area. This must be done before approaching the final boss monster of the area.
 * Ruins: 21 kills (Toriel)
 * Snowdin: 15 kills (Papyrus)
 * Waterfall: 18 kills (Undyne the Undying)
 * Hotland: 40 kills (Mettaton NEO)
 * Kill every normal boss (i.e. secret boss So Sorry is not required)
 * Toriel
 * Lesser Dog
 * Greater Dog
 * Dogamy and Dogaressa
 * Doggo
 * Papyrus
 * Mad Dummy
 * Monster Kid
 * Undyne the Undying
 * Royal Guards
 * Muffet
 * Mettaton NEO
 * Do not whiff your only attack against Mettaton NEO
 * You only get one attack, and it is impossible to miss.
 * If you would have missed, Mettaton still dies, but detects that you were holding back.
 * If this happens, you will be shunted back to the Neutral Route!

Note that Mettaton NEO is a critical character for a Genocide run. If the player is not on the Genocide Route, Alphys' scenes will proceed as usual on the Neutral Route, and they will instead fight Mettaton EX. If the player is on a Genocide Route, Alphys will never appear, the traps will be deactivated, and the player will instead fight Mettaton NEO. Mettaton is also the point of no return for the Genocide Route. Should the player kill Mettaton NEO without whiffing their attack, there is only one boss left who will kill you for sparing him. (get dunked on)

Kill counters are another tricky issue. If certain events are triggered before clearing an area completely of monsters, you will be automatically sent back to Neutral Route. For example, if you fight Papyrus without exhausting the Snowdin kill counter, the cutscene will proceed as if you were on a Neutral Route. Even if you let him capture you, go back and finish off the last monsters, you are still on the Neutral Route. Once you have left the Genocide Route, you cannot return to it.

Main Story
In the ruins, the protagonist kills the dummy in the tutorial, causing Toriel to gently berate them. The protagonist then proceeds to kill every monster in the Ruins, until nobody else comes to face them. In Toriel's Home, if the player checks the kitchen, the protagonist asks "Where are the knives."

When facing Toriel at the way out of the ruins, the protagonist kills her with a single devastating blow. She is astonished by the protagonist's hatred before she succumbs to her wound, and tells them that by keeping you there, they weren't protecting you, but everyone else. In the next chamber, Flowey identifies the player as the first human, seeing them as an empty vessel like him but with a 'stolen soul', and offers to destroy the world (and its inhabitants) together.

In Snowdin forest, the protagonist doesn't react well to Sans' pranks. He tells them that his brother, Papyrus, would really like to see a human, and, having inferred that they are a cruel person, asks the protagonist to help by pretending to be one. Afterwards the player can dismantle the snowman, getting three snowman pieces. They skip Papyrus' puzzles and arrive at the town, finding it mostly evacuated. Papyrus notices the human's behavior and offers them guidance, sparing them and leaving him vulnerable. The protagonist kills Papyrus, who tells them that they can still be a better person.

In the Waterfall, all of the echo flowers are oddly silent, as if either no one bothered to speak into them, or they somehow understand how dangerous the player is. The protagonist upsets the Mad Dummy, causing it to take a corporeal form with the dummy, allowing them to be killed. Gerson remains in his shop, aware that as he is a shopkeeper he cannot be fought, and sells his items as usual. The Monster Kid approaches the protagonist, who attacks them, but Undyne takes the hit. Instead of dying, she recovers and becomes a powerful heroine, telling the protagonist that she will save not only monsters but humans as well as the entire world by stopping them. She is defeated, but she dies with a smile, aware that Alphys is evacuating the monsters ahead of the protagonist, and will tell Asgore to absorb the souls he has collected.

In Hotland, the protagonist encounters Mettaton, who flees, and kills any other monster in their way, including Muffet. The MTT Resort is mostly abandoned, except for the MTT-Brand Burger Emporium employee, who does business as usual. The protagonist kills the monsters in the Core before leaving to fight Mettaton, who reveals his new form as Mettaton NEO before being killed with a single hit.

In New Home, Flowey talks to the player, telling them about his life ever since he was a flower, their ability to SAVE and why they started killing. Flowey reiterates his "Kill or be killed" mentality, saying that he and the player would not hesitate to kill each other if the need arises. He finally realizes, in horror, that he himself is not above this and, eventually, the protagonist will kill him; he runs away afterward. In the Final Corridor, Sans fights the protagonist, telling them that they are a threat to all the timelines of the world. He traps the player in his turn after many attacks and falls asleep, and the protagonist takes the opportunity to kill him, gaining enough EXP to reach LV 20.

In the Throne Room, the protagonist approaches Asgore, who was talking to Flowey, and automatically attacks him, depleting his health in one hit. Flowey finishes Asgore off, then, with his face and voice turning to Asriel's, he begs the protagonist not to kill him. The protagonist attacks Flowey repeatedly, killing him, and continues until there is nothing left.

Instead of leaving the black screen and returning to the Throne Room, The First Human appears in the black screen and talks to the player. They offer to destroy the world so that the two of them can move on to another game, where they will continue their slaughter. At this point, the player is given a prompt to 'ERASE' the world, or 'DO NOT'. Following either option, a larger slash animation is seen followed by the screen shaking and filled with '9's'.
 * If the player chooses 'ERASE', they will call the player 'a great partner'.
 * Choosing 'DO NOT' will cause a jumpscare, where they tell the player that they were never in control, coming closer towards the screen with a horrifying facial expression while apparently laughing as the screen shakes and flashes to red and black.

Aborting a Genocide Run
On a genocide run the player can act mercifully. This ends the genocide run and the game will continue as if on the Neutral Route.

Sparing the monster kid results in the game entering the Neutral Route, and the player will fight Undyne normally.

There is a specific neutral ending that can be gained by entering the Neutral Route at Mettaton NEO by not killing every monster in the Hotland or by striking Mettaton inaccurately. In the ending's phone call, Sans hands the phone to Alphys.

Post-Genocide Game
After having completed a genocide route, once you've chosen whether or not to erase the world, and after the corresponding dialogue, the first human strikes at the screen, destroying the world. This closes the game.

Upon a relaunch, you'll happen upon a black screen with the sound of howling wind. The inputs do nothing here. There are no menus. No buttons, nothing. The world is no more.

After waiting approximately 10 minutes in this screen, the fallen human will speak to you. Reminding you that you're the one responsible for the world's destruction and ask if you are above consequences. Regardless of your choice, they come up with a compromise: they will give you the option to restore your world, at the cost of your SOUL. If the player chooses to decline these terms, they will mock the player to remain in the abyss and leave, forcing the player to wait an additional 10 minutes to have the deal re-offered. If the trade is accepted, then the game will crash once more and your SAVE file will be deleted. You will be able to restart the game with everything restored. However, the game will be permanently altered from now on; the most notable is when you complete the Pacifist route.
 * At the end of the Pacifist ending, you find out your bargain with the first human has allowed them to take over Frisk's body:
 * If Frisk choose to stay with Toriel, after the latter leaves a piece of pie before leaving the room, they will turn over in bed in the final scene and face the camera with glowing red eyes and a more terrifying version of Flowey's laugh.
 * If Frisk says they "have places to go", the photograph of Frisk and friends shown in the final scene changes - everyone's faces except Frisk's has been crossed out with a red X, implying Chara has killed them all.
 * "The End" text changes from white to red. Also, the Annoying Dog does not show up to sleep under the text.
 * At the end of the Genocide ending:
 * Chara chastises the player for their "perverted sentimentality" and suggests that should they recreate the world again, a different course of action would be advised. (Chara wants to escape and destroy the Human world, they are encouraging the player to complete a Pacifist Route)
 * If the player selects "DO NOT", Chara will claim "You already made your choice long ago.", leading to the same cutscene like the first time.

Consequence Avoidance
To return to a playable state, the player can enter their save data folder and either:
 * Delete or rename the file "system_information_962". This will remove any effects of having done the genocide route. This file being present without "system_information_963" is what causes the game to load 'destroyed' until the player sells their soul.
 * Copy the above file or create a new file and (re)name it "system_information_963". The presence of this file will load the game normally, but with all post-genocide route effects. Essentially this file is noting you have sold your soul to the First Human.
 * Alternatively, one can completely uninstall the game and do a fresh reinstall.

Default save locations are as follows:

Windows: ☀%SystemDrive%\Users\%USERNAME%\AppData\Local\UNDERTALE

Mac: ☀~/Library/Application Support/com.tobyfox.undertale/

Alternate Method
Go to ☀ C:\Users[your username]\AppData\Local\UNDERTALE

Right click the file system_information_962 or system_information_963, depending on if you are on post-genocide black screen, or post-genocide post-soul-sold.

Go into properties, then security. Remove [Your username] and [administrators] rights to READ the file. Click apply.

When you next start up the game, it will be as if you had never done a genocide run ever before.

Demo
In the Undertale demo, once you do this, the ending screen appears with red text saying 'That was fun. Let's finish the job.' and ominous music plays - this music is the pre-battle theme slowed down significantly.

The pages of the manual (except for the first and last) will be replaced with said ending screen, and the last page will be replaced with a picture of a faceless Flowey.