Killer!Sans is the protagonist of the Something New timeline of Undertale, created by Rahafwabas. The storyline of Killer!Sans has multiple confirmed endings, although there exist many more, as the endings are left open for interpretation.

Endings

Something New AT has timeline branches with both 'good' and 'bad' endings.

• Good Endings:

• Killer!Sans overpowers Chara's determination and kills them.

• After joining the Bad Sanses, Killer!Sans meets Color!Sans who encourages Killer to live with free will and urging him not to suppress his emotions, ignoring Nightmare!Sans's commands. Color!Sans then helps Killer with deceiving Nightmare by faking the stage of his soul. The two leave together, most likely traveling together after this incident.

• Bad Endings:

  • Killer!Sans kills Chara and prepares to erase his own existence. Before he can do so, however, he gets interrupted by Nightmare!Sans who converts him into one of the Bad Sanses by manipulating his soul's stages.

  • Killer!Sans helps Chara by continuing the genocide routes.

• Other Endings:

  • Because the ending of Something New is left to interpretation, there exist many other possible endings beyond the ones listed in the original comic.

Appearance

Killer!Sans looks similar to counterpart, Classic Sans, except for his color theme being different.

Outfit: Dull/Darkened blue jacket with a fluffy hood (often depicted as having a black stain on the chest), black turtleneck, shorts with inverted colors, shoes instead of slippers (sometimes with no laces or untied ones). However, despite this, he can be seen wearing a normal outfit which Classic Sans wears, too (always during Something New timeline/comic), making it canon to draw him with the standard Sans outfit.

Physical: eye socket, usually his right one is "droopy" and half closed, black liquid dripping out (depending on stage), red soul (shape depending on stage)

Dripping determination: The black stuff often seen dripping out of Killer's eye sockets is dripping determination, how much of it is dripping is depend on stage. Higher the stage, Higher is amount of dripping determination.
Killer!Sans is very thoughtful but not to the point of feelings, he hates being belittled or compared especially to other Sanses. (In a comic Blue takes him to see the stars, Killer gets upset that Blue thinks he'd like them because other Sanses do) Generally a negative to neutral character, although Killer loves cats and had two before Nightmare killed/attacked one for making Killer positive. Killer also seems to suffer from some type of personality/identity disorder similar to DID. He'll have little breakdowns and freak outs when he's beginning to feel again and is reminded of the pain he's caused. Not only that but he also has a shadow-like hallucinations (like Papyrus, Chara, Frisk, etc) that will sometimes appear to him, possibly symbolizing his guilt going into overdrive as his sins crawl up his spine. He's hard to read and often unpredictable, his emotions are often suppressed or out of control, when they are regulated however he has a difficult time with them, often feeling numb or upset. He is often in pain. He very rarely eat food and tries to avoid Ketchup and Chocolate because it brings back memories. Because of so many resets Killer Sans sometimes can't recognize what is real or not. Hardest enemy he fights is his brother. It is possible that Killer Sans is intentionally trying to avoid his Home timeline because of his guilt. Stars are reminding him of resets.

Killer!Sans's soul has four "stages":

Stage #1: Sans is surfaced, in this stage, others can normally talk to him and he is generally approachable. In this stage, his soul resembles an upside down human soul.

Stage #2: Classic Killer, should not be trusted. His emotions are suppressed and he's unpredictable. In this stage, his soul resembles the iconic red circle on his chest.

Stage #3: "Crazy Mode", he will hurt anything and anybody in his way, completely losing hiswhether he means to or not, very violent. In this stage, his soul looks like a blurrier version of stage #2.

Stage #4: Details about this stage are vague but it seems it is most dangerous stage, and it is shown that it can get worse. In beginning of this stage Killer Sans will get overwhelmed by guilt, both his right iris and magic eye will show up, after this his mouth and eyes will become Fully black, eyes Fully open, and determination coming from his eyes will became like tears of Delta Sans but lot wider. In this stage he will ignore anyone's commands (even commands given by Nightmare). For example he will kill Papyrus despite he doesn't need to (he only needs his negative emotions, nothing else), killing Papyrus and Frisk so fast that they didn't even have time for nearly any reaction nor negative emotions, which was sole reason Killer Sans came there) and kills everything and everyone present (even if he didn't want to). In this stage only thing he will show is his scary face, and will say absolutely nothing until killing everybody present.

Note: Killer only has 4 stages in canon.

Real Knife: Gives 99 attack, this is same knife Chara was using before Killer Sans killed them. However he is sometimes Seen with Real Knife before killing Chara. Killer Sans can't create any red knifes.

Gaster Blaster: He is not using it much. His Gaster blaster is bigger than the Gaster blasters of other Sanses. Its full power is currently unknown, but it is most probably very destructive. Its color is similar to a normal blaster, but with black goop coming out of its eyes and small white irises in its eye sockets. He uses it when he is uninterested in a fight and wants to end it quickly. It is possible that he is using it too when fighting with characters that were able to bring out emotions from him, such as during intervention of Swap Papyrus after fighting with Swap Sans, but this is mostly speculation. Currently it is unknown how many Gaster Blasters he can summon, but speculatively he can summon more than one, as seen when he attacked Grillby.

Bones: His bones are often very sharp, but has normal Bones too. Size of bones can vary.

Blue Bones: Killer has Blue Bones too.

Soul Manipulation: Blue Mode: Sans can can make enemy souls affected by gravity While affected by blue mode, enemies have their movement heavily restricted, and are only able to jump to move.

High LV: 99999 or 20. (Usually 20)

Determination: Killer is able to RESET. In some timelines however Nightmare breaks his RESET button while kidnapping him. ( Note: If rules are same AS in Undertale, he Can SAVE and LOAD too.)

Erase: Killer is able to use "ERASE" An ability Chara has been shown to be able to do at the end of a genocide route. Circumstances to use this ability are unknown.

Telekinesis: Like normal Sans, Killer Sans can use telekinesis.

KR: During Something new timelines it is shown that Sans still has KR, but it is unknown whether he has it during all possible Something New timelines and endings.

Teleportation: He can teleport like Undertale Sans, nothing new.

Dodging: He can dodge like Undertale Sans, nothing new.

Insanity: sometimes, examples: Stage 3

Determination Resistance: He has high DT allowing him to be partially invulnerable to other DETERMINATION related abilities (like Flowey).

Strength: While not seen very much, he is physically strong too, for example breaking Rib Bone (one of the strongest bones) of Swap Sans by hand and punching Ink Sans far away.

Intelligence: While sometimes portrayed as dumb, he is pretty intelligent. He doesn't attack characters who he can't beat, and instead waits for right time to make a move (Nightmare/Chara). He knows that a character's strength is based on how strong "Players" view them, and can't beat them unless he is in an AU where "Players" view him stronger than his opponent (Example: He warns Nightmare that he can't beat Core!Frisk in an AU they are in, and instead must find an AU where "Players" view him stronger than Core!Frisk is). He is collecting dust, blood and souls to research them, in case of monster souls he uses telekinesis to hold that soul together. (This is information based on official comics, go here for the comics, times: [8:08-8:28] and [8:48-8:49]. (This is temporary reference link, this will get eventually replaced by the official links).

Hallucinations: He has hallucinations of people he killed (most commonly Chara and Frisk. For most time he knows they are not real and if he somehow forgot, they will even remind him that. Most commonly they are found giving him advice. Third most common hallucination is that of Papyrus). "He is sometimes sick for killing Papyrus".