Killer Sans

Staging as a mechanic: Killer's stages aren't just visual changes, but a full-fledged system that affects his strength, sanity, and fighting style.

Soul Distortion: The shape and color of the soul emphasize that he has ceased to be "normal" Sans and has become something else - an instrument of destruction.

Contrast with Dusttale Sans: while Dust!Sans acts out of a sense of duty and guilt ("I must stop evil"), Killer acts out of a desire to feel/destroy or for profit; his path is the disintegration of personality for the sake of power and emotion.

Chara: a partner in a deal, then a victim; Chara's murder is interpreted as both a betrayal, a way to "take power," and a final gesture of rejecting someone else's rules.

Nightmare Sans: Killer's boss and handler; he carries out his orders but maintains a certain independence and a penchant for excessive cruelty.

Papyrus: In most versions, Papyrus is killed by Killer; attitudes towards him range from cold calculation to a repressed sense of guilt that he deliberately suppresses.

Other Sanses (Dust, Fell, Swapfell, Horror, etc.): often found on the same "bad team" with them, but due to his instability, he can conflict even with allies.

Before the "deal": lazy, sarcastic, with a habit of joking, but already emotionally burned out due to constant reboots.

Post-deal: increasing loss of empathy, shifting from “I do it to feel” to “I enjoy causing pain.”

Stages: Behavior depends heavily on the current stage. In the early stages, he is still capable of irony and even limited self-control; in later stages, he becomes unpredictable and extremely dangerous.

Internal conflict: In some interpretations, a shadow of the old Sans remains, struggling with the monster within; in others, he fully embraces his new role.

Combat characteristics

Gaster Blasters: Powerful energy attacks that increase in power as they progress through stages.

Osteokinesis (Bone Control): Creates sharp, fast, and unpredictable attacks; can change the shape and trajectory of bones.

Teleportation and evasion: instant movement, high reaction, the ability to avoid attacks and change position in battle.

KR (Karmic Retribution) / damage effects: can apply special effects that affect not only the enemy's health, but also their mechanics (in fan lore, this is often associated with code/timeline corruption).

Knowledge of AUs and Weaknesses: In some versions, Killer has knowledge of various alternate universes, their vulnerabilities, and methods of destruction.

Tactics: aggressive, chaotic, with an emphasis on psychological pressure, surprise attacks and the destruction of the enemy's control over the situation.

Eyes and "stages": Killer Sans has several stages of madness/power, which are reflected in his appearance:
Stage 1: Almost like normal Sans, minimal changes.
Stages 2-3: a substance begins to leak from the eye sockets (often interpreted as “an excess of determination”), the look becomes frightening, and the facial features become distorted.
Stage 4: Extreme instability: face and mouth lose clear outlines, “blurred” physiology effect, maximum danger.

Soul: Varies depending on the stage; often depicted as a red target or an inverted heart with a thick outline, emphasizing its "target" and twisted nature.

Clothes: Jacket with a fur hood (slightly off-blue, closer to a gray-blue), black shirt, fingerless gloves, black and white striped shorts (an inverted version of Sans's classic shorts), blue sneakers.

Key Event: After numerous genocide routes and timeline resets, Chara offers Sans a deal: kill together to "feel something." He initially rejects it, but eventually breaks down and accepts.

Turning Point: At some point, Killer kills Chara, taking the accumulated Violence Level (LV) for himself, and becomes a threat in his own right.

What happens next: Nightmare Sans finds him and recruits him into the ranks of the "bad Sanses" - a team of multiverse destroyers.
Biographical information

Motivation: initially, an attempt to escape the emotional void and "feel" something; later, pure enjoyment of chaos, or following Nightmare's orders. The degree of his awareness and "evil will" varies in different interpretations.